Show that base is twice the height if base angles of a triangle are $22.5^circ$ and $112.5^circ$












1












$begingroup$



The base angles of a triangle are $22.5^circ$ and $112.5^circ$. Show that the base is twice the height.




My Attempt
enter image description here



$$
h=c.sin22.5^circ=c.cos 67.5^circ\
=bsin 67.5^circ=bcos 22.5^circ
$$

$$
a=ccos22.5^circ- bsin22.5circ=frac{h}{b}-frac{h}{c}=hcdotfrac{c-b}{bc}
$$



I have no clue of how to prove this.










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  • $begingroup$
    You should have pointed upon every vertex.
    $endgroup$
    – Rakibul Islam Prince
    Jan 1 at 23:15






  • 1




    $begingroup$
    @RakibulIslamPrince Fixed.
    $endgroup$
    – A.Γ.
    Jan 1 at 23:51
















1












$begingroup$



The base angles of a triangle are $22.5^circ$ and $112.5^circ$. Show that the base is twice the height.




My Attempt
enter image description here



$$
h=c.sin22.5^circ=c.cos 67.5^circ\
=bsin 67.5^circ=bcos 22.5^circ
$$

$$
a=ccos22.5^circ- bsin22.5circ=frac{h}{b}-frac{h}{c}=hcdotfrac{c-b}{bc}
$$



I have no clue of how to prove this.










share|cite|improve this question











$endgroup$












  • $begingroup$
    You should have pointed upon every vertex.
    $endgroup$
    – Rakibul Islam Prince
    Jan 1 at 23:15






  • 1




    $begingroup$
    @RakibulIslamPrince Fixed.
    $endgroup$
    – A.Γ.
    Jan 1 at 23:51














1












1








1


0



$begingroup$



The base angles of a triangle are $22.5^circ$ and $112.5^circ$. Show that the base is twice the height.




My Attempt
enter image description here



$$
h=c.sin22.5^circ=c.cos 67.5^circ\
=bsin 67.5^circ=bcos 22.5^circ
$$

$$
a=ccos22.5^circ- bsin22.5circ=frac{h}{b}-frac{h}{c}=hcdotfrac{c-b}{bc}
$$



I have no clue of how to prove this.










share|cite|improve this question











$endgroup$





The base angles of a triangle are $22.5^circ$ and $112.5^circ$. Show that the base is twice the height.




My Attempt
enter image description here



$$
h=c.sin22.5^circ=c.cos 67.5^circ\
=bsin 67.5^circ=bcos 22.5^circ
$$

$$
a=ccos22.5^circ- bsin22.5circ=frac{h}{b}-frac{h}{c}=hcdotfrac{c-b}{bc}
$$



I have no clue of how to prove this.







trigonometry triangle






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edited Jan 1 at 23:50









A.Γ.

22.7k32656




22.7k32656










asked Jan 1 at 22:48









ss1729ss1729

1,8991723




1,8991723












  • $begingroup$
    You should have pointed upon every vertex.
    $endgroup$
    – Rakibul Islam Prince
    Jan 1 at 23:15






  • 1




    $begingroup$
    @RakibulIslamPrince Fixed.
    $endgroup$
    – A.Γ.
    Jan 1 at 23:51


















  • $begingroup$
    You should have pointed upon every vertex.
    $endgroup$
    – Rakibul Islam Prince
    Jan 1 at 23:15






  • 1




    $begingroup$
    @RakibulIslamPrince Fixed.
    $endgroup$
    – A.Γ.
    Jan 1 at 23:51
















$begingroup$
You should have pointed upon every vertex.
$endgroup$
– Rakibul Islam Prince
Jan 1 at 23:15




$begingroup$
You should have pointed upon every vertex.
$endgroup$
– Rakibul Islam Prince
Jan 1 at 23:15




1




1




$begingroup$
@RakibulIslamPrince Fixed.
$endgroup$
– A.Γ.
Jan 1 at 23:51




$begingroup$
@RakibulIslamPrince Fixed.
$endgroup$
– A.Γ.
Jan 1 at 23:51










5 Answers
5






active

oldest

votes


















4












$begingroup$

By the law of sines, $$frac{a}{sin{45^{circ}}}=frac{b}{sin{22.5^{circ}}}$$



By the double angle formula, this is equivalent to $$frac{a}{2sin{22.5^{circ}}cos{22.5^{circ}}}=frac{b}{sin{22.5^{circ}}}impliesfrac{a}{2cos{22.5^{circ}}}=b$$



From the smaller right triangle we see that $$frac{h}{b}=cos{22.5^{circ}}implies h=bcos{22.5^{circ}}$$



Combining the results gives $a=2h$.






share|cite|improve this answer









$endgroup$





















    2












    $begingroup$

    The two right angled triangles in your picture are similar and both have smaller angle $22.5$. Let the shortest unmarked side be $x$



    Then $$frac{h}{a+x}=frac xh=tan22.5=sqrt{2}-1$$



    Eliminating $x$ gives $$h^2=ah(sqrt{2}-1)+h^2(sqrt{2}-1)^2$$
    Rearranging gives $$frac ah=frac{2sqrt{2}-2}{sqrt{2}-1}=2$$






    share|cite|improve this answer









    $endgroup$





















      1












      $begingroup$

      Let,the extended portion of $a=x$ and let $D$ be the intersection point of base and height.



      So,$CD=x$



      Now,in the $triangle ABD$,
      $$tan (22.5)=frac{h}{a+x}implies h=atan(22.5)+xtan(22.5)......(1)$$
      in the $triangle ACD$,
      $$tan (67.5)=frac{h}{x}implies h=xtan (67.5)implies x=frac{h}{tan (67.5)}$$
      from (1),
      $$h=atan(22.5)+frac{h}{tan (67.5)}tan(22.5)$$
      $$implies h=frac{a}{2}[text{after simplification}]$$






      share|cite|improve this answer











      $endgroup$





















        0












        $begingroup$

        The base of the triangle is $ccos(22.5)-bcos(180-112.5)$. The height is $csin(22.5)$. Also, $bsin(180-112.5) = csin(22.5)$. Use these two relations to write $b$ in terms of $c$ and show that $$frac{csin(22.5)}{ccos(22.5)-bcos(180-112.5)} = 1/2.$$ (Hint: the $c$ will cancel out).






        share|cite|improve this answer









        $endgroup$





















          0












          $begingroup$

          Solution without trigonometry.



          Since $$measuredangle CAD=measuredangle BCD-90^{circ}=112.5^{circ}-90^{circ}=22.5^{circ}=measuredangle ABD,$$
          we get that $DA$ is a tangent line to the circumcircle of $Delta ABC$.



          Let $O$ be a center of the circle and $OM$ be an altitude of $Delta OBC$.



          Since $OAperp DA$, we obtain $$measuredangle $OCM=measuredangle ABC-measuredangle OCA=measuredangle ABC-measuredangle OAC=112.5^{circ}-(90^{circ}-22.5^{circ})=45^{circ},$$
          which says $OM=MC$ and since $BM=MC,$ we obtain:
          $$BC=2OM=2AD$$ and we are done!






          share|cite|improve this answer









          $endgroup$













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            5 Answers
            5






            active

            oldest

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            5 Answers
            5






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

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            4












            $begingroup$

            By the law of sines, $$frac{a}{sin{45^{circ}}}=frac{b}{sin{22.5^{circ}}}$$



            By the double angle formula, this is equivalent to $$frac{a}{2sin{22.5^{circ}}cos{22.5^{circ}}}=frac{b}{sin{22.5^{circ}}}impliesfrac{a}{2cos{22.5^{circ}}}=b$$



            From the smaller right triangle we see that $$frac{h}{b}=cos{22.5^{circ}}implies h=bcos{22.5^{circ}}$$



            Combining the results gives $a=2h$.






            share|cite|improve this answer









            $endgroup$


















              4












              $begingroup$

              By the law of sines, $$frac{a}{sin{45^{circ}}}=frac{b}{sin{22.5^{circ}}}$$



              By the double angle formula, this is equivalent to $$frac{a}{2sin{22.5^{circ}}cos{22.5^{circ}}}=frac{b}{sin{22.5^{circ}}}impliesfrac{a}{2cos{22.5^{circ}}}=b$$



              From the smaller right triangle we see that $$frac{h}{b}=cos{22.5^{circ}}implies h=bcos{22.5^{circ}}$$



              Combining the results gives $a=2h$.






              share|cite|improve this answer









              $endgroup$
















                4












                4








                4





                $begingroup$

                By the law of sines, $$frac{a}{sin{45^{circ}}}=frac{b}{sin{22.5^{circ}}}$$



                By the double angle formula, this is equivalent to $$frac{a}{2sin{22.5^{circ}}cos{22.5^{circ}}}=frac{b}{sin{22.5^{circ}}}impliesfrac{a}{2cos{22.5^{circ}}}=b$$



                From the smaller right triangle we see that $$frac{h}{b}=cos{22.5^{circ}}implies h=bcos{22.5^{circ}}$$



                Combining the results gives $a=2h$.






                share|cite|improve this answer









                $endgroup$



                By the law of sines, $$frac{a}{sin{45^{circ}}}=frac{b}{sin{22.5^{circ}}}$$



                By the double angle formula, this is equivalent to $$frac{a}{2sin{22.5^{circ}}cos{22.5^{circ}}}=frac{b}{sin{22.5^{circ}}}impliesfrac{a}{2cos{22.5^{circ}}}=b$$



                From the smaller right triangle we see that $$frac{h}{b}=cos{22.5^{circ}}implies h=bcos{22.5^{circ}}$$



                Combining the results gives $a=2h$.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Jan 1 at 23:31









                John DoumaJohn Douma

                5,53211319




                5,53211319























                    2












                    $begingroup$

                    The two right angled triangles in your picture are similar and both have smaller angle $22.5$. Let the shortest unmarked side be $x$



                    Then $$frac{h}{a+x}=frac xh=tan22.5=sqrt{2}-1$$



                    Eliminating $x$ gives $$h^2=ah(sqrt{2}-1)+h^2(sqrt{2}-1)^2$$
                    Rearranging gives $$frac ah=frac{2sqrt{2}-2}{sqrt{2}-1}=2$$






                    share|cite|improve this answer









                    $endgroup$


















                      2












                      $begingroup$

                      The two right angled triangles in your picture are similar and both have smaller angle $22.5$. Let the shortest unmarked side be $x$



                      Then $$frac{h}{a+x}=frac xh=tan22.5=sqrt{2}-1$$



                      Eliminating $x$ gives $$h^2=ah(sqrt{2}-1)+h^2(sqrt{2}-1)^2$$
                      Rearranging gives $$frac ah=frac{2sqrt{2}-2}{sqrt{2}-1}=2$$






                      share|cite|improve this answer









                      $endgroup$
















                        2












                        2








                        2





                        $begingroup$

                        The two right angled triangles in your picture are similar and both have smaller angle $22.5$. Let the shortest unmarked side be $x$



                        Then $$frac{h}{a+x}=frac xh=tan22.5=sqrt{2}-1$$



                        Eliminating $x$ gives $$h^2=ah(sqrt{2}-1)+h^2(sqrt{2}-1)^2$$
                        Rearranging gives $$frac ah=frac{2sqrt{2}-2}{sqrt{2}-1}=2$$






                        share|cite|improve this answer









                        $endgroup$



                        The two right angled triangles in your picture are similar and both have smaller angle $22.5$. Let the shortest unmarked side be $x$



                        Then $$frac{h}{a+x}=frac xh=tan22.5=sqrt{2}-1$$



                        Eliminating $x$ gives $$h^2=ah(sqrt{2}-1)+h^2(sqrt{2}-1)^2$$
                        Rearranging gives $$frac ah=frac{2sqrt{2}-2}{sqrt{2}-1}=2$$







                        share|cite|improve this answer












                        share|cite|improve this answer



                        share|cite|improve this answer










                        answered Jan 1 at 23:47









                        David QuinnDavid Quinn

                        23.9k21141




                        23.9k21141























                            1












                            $begingroup$

                            Let,the extended portion of $a=x$ and let $D$ be the intersection point of base and height.



                            So,$CD=x$



                            Now,in the $triangle ABD$,
                            $$tan (22.5)=frac{h}{a+x}implies h=atan(22.5)+xtan(22.5)......(1)$$
                            in the $triangle ACD$,
                            $$tan (67.5)=frac{h}{x}implies h=xtan (67.5)implies x=frac{h}{tan (67.5)}$$
                            from (1),
                            $$h=atan(22.5)+frac{h}{tan (67.5)}tan(22.5)$$
                            $$implies h=frac{a}{2}[text{after simplification}]$$






                            share|cite|improve this answer











                            $endgroup$


















                              1












                              $begingroup$

                              Let,the extended portion of $a=x$ and let $D$ be the intersection point of base and height.



                              So,$CD=x$



                              Now,in the $triangle ABD$,
                              $$tan (22.5)=frac{h}{a+x}implies h=atan(22.5)+xtan(22.5)......(1)$$
                              in the $triangle ACD$,
                              $$tan (67.5)=frac{h}{x}implies h=xtan (67.5)implies x=frac{h}{tan (67.5)}$$
                              from (1),
                              $$h=atan(22.5)+frac{h}{tan (67.5)}tan(22.5)$$
                              $$implies h=frac{a}{2}[text{after simplification}]$$






                              share|cite|improve this answer











                              $endgroup$
















                                1












                                1








                                1





                                $begingroup$

                                Let,the extended portion of $a=x$ and let $D$ be the intersection point of base and height.



                                So,$CD=x$



                                Now,in the $triangle ABD$,
                                $$tan (22.5)=frac{h}{a+x}implies h=atan(22.5)+xtan(22.5)......(1)$$
                                in the $triangle ACD$,
                                $$tan (67.5)=frac{h}{x}implies h=xtan (67.5)implies x=frac{h}{tan (67.5)}$$
                                from (1),
                                $$h=atan(22.5)+frac{h}{tan (67.5)}tan(22.5)$$
                                $$implies h=frac{a}{2}[text{after simplification}]$$






                                share|cite|improve this answer











                                $endgroup$



                                Let,the extended portion of $a=x$ and let $D$ be the intersection point of base and height.



                                So,$CD=x$



                                Now,in the $triangle ABD$,
                                $$tan (22.5)=frac{h}{a+x}implies h=atan(22.5)+xtan(22.5)......(1)$$
                                in the $triangle ACD$,
                                $$tan (67.5)=frac{h}{x}implies h=xtan (67.5)implies x=frac{h}{tan (67.5)}$$
                                from (1),
                                $$h=atan(22.5)+frac{h}{tan (67.5)}tan(22.5)$$
                                $$implies h=frac{a}{2}[text{after simplification}]$$







                                share|cite|improve this answer














                                share|cite|improve this answer



                                share|cite|improve this answer








                                edited Jan 1 at 23:12

























                                answered Jan 1 at 23:06









                                Rakibul Islam PrinceRakibul Islam Prince

                                1,008211




                                1,008211























                                    0












                                    $begingroup$

                                    The base of the triangle is $ccos(22.5)-bcos(180-112.5)$. The height is $csin(22.5)$. Also, $bsin(180-112.5) = csin(22.5)$. Use these two relations to write $b$ in terms of $c$ and show that $$frac{csin(22.5)}{ccos(22.5)-bcos(180-112.5)} = 1/2.$$ (Hint: the $c$ will cancel out).






                                    share|cite|improve this answer









                                    $endgroup$


















                                      0












                                      $begingroup$

                                      The base of the triangle is $ccos(22.5)-bcos(180-112.5)$. The height is $csin(22.5)$. Also, $bsin(180-112.5) = csin(22.5)$. Use these two relations to write $b$ in terms of $c$ and show that $$frac{csin(22.5)}{ccos(22.5)-bcos(180-112.5)} = 1/2.$$ (Hint: the $c$ will cancel out).






                                      share|cite|improve this answer









                                      $endgroup$
















                                        0












                                        0








                                        0





                                        $begingroup$

                                        The base of the triangle is $ccos(22.5)-bcos(180-112.5)$. The height is $csin(22.5)$. Also, $bsin(180-112.5) = csin(22.5)$. Use these two relations to write $b$ in terms of $c$ and show that $$frac{csin(22.5)}{ccos(22.5)-bcos(180-112.5)} = 1/2.$$ (Hint: the $c$ will cancel out).






                                        share|cite|improve this answer









                                        $endgroup$



                                        The base of the triangle is $ccos(22.5)-bcos(180-112.5)$. The height is $csin(22.5)$. Also, $bsin(180-112.5) = csin(22.5)$. Use these two relations to write $b$ in terms of $c$ and show that $$frac{csin(22.5)}{ccos(22.5)-bcos(180-112.5)} = 1/2.$$ (Hint: the $c$ will cancel out).







                                        share|cite|improve this answer












                                        share|cite|improve this answer



                                        share|cite|improve this answer










                                        answered Jan 1 at 22:56









                                        D.B.D.B.

                                        1,2128




                                        1,2128























                                            0












                                            $begingroup$

                                            Solution without trigonometry.



                                            Since $$measuredangle CAD=measuredangle BCD-90^{circ}=112.5^{circ}-90^{circ}=22.5^{circ}=measuredangle ABD,$$
                                            we get that $DA$ is a tangent line to the circumcircle of $Delta ABC$.



                                            Let $O$ be a center of the circle and $OM$ be an altitude of $Delta OBC$.



                                            Since $OAperp DA$, we obtain $$measuredangle $OCM=measuredangle ABC-measuredangle OCA=measuredangle ABC-measuredangle OAC=112.5^{circ}-(90^{circ}-22.5^{circ})=45^{circ},$$
                                            which says $OM=MC$ and since $BM=MC,$ we obtain:
                                            $$BC=2OM=2AD$$ and we are done!






                                            share|cite|improve this answer









                                            $endgroup$


















                                              0












                                              $begingroup$

                                              Solution without trigonometry.



                                              Since $$measuredangle CAD=measuredangle BCD-90^{circ}=112.5^{circ}-90^{circ}=22.5^{circ}=measuredangle ABD,$$
                                              we get that $DA$ is a tangent line to the circumcircle of $Delta ABC$.



                                              Let $O$ be a center of the circle and $OM$ be an altitude of $Delta OBC$.



                                              Since $OAperp DA$, we obtain $$measuredangle $OCM=measuredangle ABC-measuredangle OCA=measuredangle ABC-measuredangle OAC=112.5^{circ}-(90^{circ}-22.5^{circ})=45^{circ},$$
                                              which says $OM=MC$ and since $BM=MC,$ we obtain:
                                              $$BC=2OM=2AD$$ and we are done!






                                              share|cite|improve this answer









                                              $endgroup$
















                                                0












                                                0








                                                0





                                                $begingroup$

                                                Solution without trigonometry.



                                                Since $$measuredangle CAD=measuredangle BCD-90^{circ}=112.5^{circ}-90^{circ}=22.5^{circ}=measuredangle ABD,$$
                                                we get that $DA$ is a tangent line to the circumcircle of $Delta ABC$.



                                                Let $O$ be a center of the circle and $OM$ be an altitude of $Delta OBC$.



                                                Since $OAperp DA$, we obtain $$measuredangle $OCM=measuredangle ABC-measuredangle OCA=measuredangle ABC-measuredangle OAC=112.5^{circ}-(90^{circ}-22.5^{circ})=45^{circ},$$
                                                which says $OM=MC$ and since $BM=MC,$ we obtain:
                                                $$BC=2OM=2AD$$ and we are done!






                                                share|cite|improve this answer









                                                $endgroup$



                                                Solution without trigonometry.



                                                Since $$measuredangle CAD=measuredangle BCD-90^{circ}=112.5^{circ}-90^{circ}=22.5^{circ}=measuredangle ABD,$$
                                                we get that $DA$ is a tangent line to the circumcircle of $Delta ABC$.



                                                Let $O$ be a center of the circle and $OM$ be an altitude of $Delta OBC$.



                                                Since $OAperp DA$, we obtain $$measuredangle $OCM=measuredangle ABC-measuredangle OCA=measuredangle ABC-measuredangle OAC=112.5^{circ}-(90^{circ}-22.5^{circ})=45^{circ},$$
                                                which says $OM=MC$ and since $BM=MC,$ we obtain:
                                                $$BC=2OM=2AD$$ and we are done!







                                                share|cite|improve this answer












                                                share|cite|improve this answer



                                                share|cite|improve this answer










                                                answered Jan 2 at 4:31









                                                Michael RozenbergMichael Rozenberg

                                                99k1590189




                                                99k1590189






























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