Area of a triangle using vectors
$begingroup$
I have to find the area of a triangle whose vertices have coordinates
O$(0,0,0)$, A$(1,-5,-7)$ and B$(10,10,5)$
I thought that perhaps I should use the dot product to find the angle between the lines $vec{OA}$ and $vec{OB}$ and use this angle in the formula:
area $= frac{1}{2}absin{C}$
These are my steps for doing this:
$mathbf{a} cdot mathbf{b} = begin{vmatrix} {mathbf{a}} end{vmatrix}begin{vmatrix} {mathbf{b}} end{vmatrix} sin{theta} $
Let $mathbf{a} = begin{pmatrix} 1 \ -5 \ -7 end{pmatrix}$ and let $mathbf{b} = begin{pmatrix} 10 \ 10 \ 5 end{pmatrix}$
$therefore begin{pmatrix} 1 \ -5 \ -7 end{pmatrix} cdot begin{pmatrix} 10 \ 10 \ 5 end{pmatrix} = (5sqrt{3})(15)sin{theta} $
$therefore sin{theta} = -dfrac{1}{sqrt{3}}$
If I substitute these values into the general formula:
area $= frac{1}{2}absin{C}$
I get:
area $= frac{1}{2}(5sqrt{3})(15)(-dfrac{1}{sqrt{3}})$
$therefore$ area $= -dfrac{75}{2}$
However this isn't right, the area should be $dfrac{75}{sqrt{2}}$
I feel I'm missing something really obvious but I can't spot it, can anyone help?
Thank you.
trigonometry triangle area
$endgroup$
add a comment |
$begingroup$
I have to find the area of a triangle whose vertices have coordinates
O$(0,0,0)$, A$(1,-5,-7)$ and B$(10,10,5)$
I thought that perhaps I should use the dot product to find the angle between the lines $vec{OA}$ and $vec{OB}$ and use this angle in the formula:
area $= frac{1}{2}absin{C}$
These are my steps for doing this:
$mathbf{a} cdot mathbf{b} = begin{vmatrix} {mathbf{a}} end{vmatrix}begin{vmatrix} {mathbf{b}} end{vmatrix} sin{theta} $
Let $mathbf{a} = begin{pmatrix} 1 \ -5 \ -7 end{pmatrix}$ and let $mathbf{b} = begin{pmatrix} 10 \ 10 \ 5 end{pmatrix}$
$therefore begin{pmatrix} 1 \ -5 \ -7 end{pmatrix} cdot begin{pmatrix} 10 \ 10 \ 5 end{pmatrix} = (5sqrt{3})(15)sin{theta} $
$therefore sin{theta} = -dfrac{1}{sqrt{3}}$
If I substitute these values into the general formula:
area $= frac{1}{2}absin{C}$
I get:
area $= frac{1}{2}(5sqrt{3})(15)(-dfrac{1}{sqrt{3}})$
$therefore$ area $= -dfrac{75}{2}$
However this isn't right, the area should be $dfrac{75}{sqrt{2}}$
I feel I'm missing something really obvious but I can't spot it, can anyone help?
Thank you.
trigonometry triangle area
$endgroup$
1
$begingroup$
$mathbf{a}cdot mathbf{b}=|mathbf{a}||mathbf{b}| cos theta$.
$endgroup$
– Meow
Feb 7 '14 at 16:54
$begingroup$
Haha, thank you, can't believe I didn't check that.
$endgroup$
– Elise
Feb 7 '14 at 16:55
2
$begingroup$
Use the cross product ...
$endgroup$
– Mark Bennet
Feb 7 '14 at 17:31
$begingroup$
@MarkBennet that would make it a lot more efficient, thank you :)
$endgroup$
– Elise
Feb 7 '14 at 18:05
add a comment |
$begingroup$
I have to find the area of a triangle whose vertices have coordinates
O$(0,0,0)$, A$(1,-5,-7)$ and B$(10,10,5)$
I thought that perhaps I should use the dot product to find the angle between the lines $vec{OA}$ and $vec{OB}$ and use this angle in the formula:
area $= frac{1}{2}absin{C}$
These are my steps for doing this:
$mathbf{a} cdot mathbf{b} = begin{vmatrix} {mathbf{a}} end{vmatrix}begin{vmatrix} {mathbf{b}} end{vmatrix} sin{theta} $
Let $mathbf{a} = begin{pmatrix} 1 \ -5 \ -7 end{pmatrix}$ and let $mathbf{b} = begin{pmatrix} 10 \ 10 \ 5 end{pmatrix}$
$therefore begin{pmatrix} 1 \ -5 \ -7 end{pmatrix} cdot begin{pmatrix} 10 \ 10 \ 5 end{pmatrix} = (5sqrt{3})(15)sin{theta} $
$therefore sin{theta} = -dfrac{1}{sqrt{3}}$
If I substitute these values into the general formula:
area $= frac{1}{2}absin{C}$
I get:
area $= frac{1}{2}(5sqrt{3})(15)(-dfrac{1}{sqrt{3}})$
$therefore$ area $= -dfrac{75}{2}$
However this isn't right, the area should be $dfrac{75}{sqrt{2}}$
I feel I'm missing something really obvious but I can't spot it, can anyone help?
Thank you.
trigonometry triangle area
$endgroup$
I have to find the area of a triangle whose vertices have coordinates
O$(0,0,0)$, A$(1,-5,-7)$ and B$(10,10,5)$
I thought that perhaps I should use the dot product to find the angle between the lines $vec{OA}$ and $vec{OB}$ and use this angle in the formula:
area $= frac{1}{2}absin{C}$
These are my steps for doing this:
$mathbf{a} cdot mathbf{b} = begin{vmatrix} {mathbf{a}} end{vmatrix}begin{vmatrix} {mathbf{b}} end{vmatrix} sin{theta} $
Let $mathbf{a} = begin{pmatrix} 1 \ -5 \ -7 end{pmatrix}$ and let $mathbf{b} = begin{pmatrix} 10 \ 10 \ 5 end{pmatrix}$
$therefore begin{pmatrix} 1 \ -5 \ -7 end{pmatrix} cdot begin{pmatrix} 10 \ 10 \ 5 end{pmatrix} = (5sqrt{3})(15)sin{theta} $
$therefore sin{theta} = -dfrac{1}{sqrt{3}}$
If I substitute these values into the general formula:
area $= frac{1}{2}absin{C}$
I get:
area $= frac{1}{2}(5sqrt{3})(15)(-dfrac{1}{sqrt{3}})$
$therefore$ area $= -dfrac{75}{2}$
However this isn't right, the area should be $dfrac{75}{sqrt{2}}$
I feel I'm missing something really obvious but I can't spot it, can anyone help?
Thank you.
trigonometry triangle area
trigonometry triangle area
asked Feb 7 '14 at 16:50
EliseElise
395314
395314
1
$begingroup$
$mathbf{a}cdot mathbf{b}=|mathbf{a}||mathbf{b}| cos theta$.
$endgroup$
– Meow
Feb 7 '14 at 16:54
$begingroup$
Haha, thank you, can't believe I didn't check that.
$endgroup$
– Elise
Feb 7 '14 at 16:55
2
$begingroup$
Use the cross product ...
$endgroup$
– Mark Bennet
Feb 7 '14 at 17:31
$begingroup$
@MarkBennet that would make it a lot more efficient, thank you :)
$endgroup$
– Elise
Feb 7 '14 at 18:05
add a comment |
1
$begingroup$
$mathbf{a}cdot mathbf{b}=|mathbf{a}||mathbf{b}| cos theta$.
$endgroup$
– Meow
Feb 7 '14 at 16:54
$begingroup$
Haha, thank you, can't believe I didn't check that.
$endgroup$
– Elise
Feb 7 '14 at 16:55
2
$begingroup$
Use the cross product ...
$endgroup$
– Mark Bennet
Feb 7 '14 at 17:31
$begingroup$
@MarkBennet that would make it a lot more efficient, thank you :)
$endgroup$
– Elise
Feb 7 '14 at 18:05
1
1
$begingroup$
$mathbf{a}cdot mathbf{b}=|mathbf{a}||mathbf{b}| cos theta$.
$endgroup$
– Meow
Feb 7 '14 at 16:54
$begingroup$
$mathbf{a}cdot mathbf{b}=|mathbf{a}||mathbf{b}| cos theta$.
$endgroup$
– Meow
Feb 7 '14 at 16:54
$begingroup$
Haha, thank you, can't believe I didn't check that.
$endgroup$
– Elise
Feb 7 '14 at 16:55
$begingroup$
Haha, thank you, can't believe I didn't check that.
$endgroup$
– Elise
Feb 7 '14 at 16:55
2
2
$begingroup$
Use the cross product ...
$endgroup$
– Mark Bennet
Feb 7 '14 at 17:31
$begingroup$
Use the cross product ...
$endgroup$
– Mark Bennet
Feb 7 '14 at 17:31
$begingroup$
@MarkBennet that would make it a lot more efficient, thank you :)
$endgroup$
– Elise
Feb 7 '14 at 18:05
$begingroup$
@MarkBennet that would make it a lot more efficient, thank you :)
$endgroup$
– Elise
Feb 7 '14 at 18:05
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
The correct formula is $mathbf{a} cdot mathbf{b} = begin{vmatrix} {mathbf{a}} end{vmatrix}begin{vmatrix} {mathbf{b}} end{vmatrix} cos{theta} $
So what you really have is $cos{theta} = cfrac{-1}{sqrt{3}}$
Therefore $$sin{theta} = sqrt{1 - cos^2{theta}} = sqrt{1 - frac{1}{3}} = sqrt{frac{2}{3}} = frac{sqrt{2}}{sqrt{3}}$$
Finally, the area of the triangle is:
$$
Area = frac{1}{2} (5 sqrt{3}) (15) frac{sqrt{2}}{sqrt{3}} = frac{75 sqrt{2}}{2}
$$
We can just multiply $frac{sqrt{2}}{sqrt{2}}$ to the area, and then we get the answer you posted:
$$
Area = frac{75 sqrt{2}}{2} left(frac{sqrt{2}}{sqrt{2}}right) = frac{75}{sqrt{2}}
$$
$endgroup$
add a comment |
$begingroup$
Since your vectors are in $mathbb{R}^3$, you can find the area of the parallelogram generated by the vectors by computing the magnitude of the cross product. The area of the triangle is half that value:
$Area=(1/2) | a times b |$.
$endgroup$
$begingroup$
Alternative approach is en.wikipedia.org/wiki/Heron%27s_formula
$endgroup$
– user2661923
Dec 31 '18 at 14:23
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
The correct formula is $mathbf{a} cdot mathbf{b} = begin{vmatrix} {mathbf{a}} end{vmatrix}begin{vmatrix} {mathbf{b}} end{vmatrix} cos{theta} $
So what you really have is $cos{theta} = cfrac{-1}{sqrt{3}}$
Therefore $$sin{theta} = sqrt{1 - cos^2{theta}} = sqrt{1 - frac{1}{3}} = sqrt{frac{2}{3}} = frac{sqrt{2}}{sqrt{3}}$$
Finally, the area of the triangle is:
$$
Area = frac{1}{2} (5 sqrt{3}) (15) frac{sqrt{2}}{sqrt{3}} = frac{75 sqrt{2}}{2}
$$
We can just multiply $frac{sqrt{2}}{sqrt{2}}$ to the area, and then we get the answer you posted:
$$
Area = frac{75 sqrt{2}}{2} left(frac{sqrt{2}}{sqrt{2}}right) = frac{75}{sqrt{2}}
$$
$endgroup$
add a comment |
$begingroup$
The correct formula is $mathbf{a} cdot mathbf{b} = begin{vmatrix} {mathbf{a}} end{vmatrix}begin{vmatrix} {mathbf{b}} end{vmatrix} cos{theta} $
So what you really have is $cos{theta} = cfrac{-1}{sqrt{3}}$
Therefore $$sin{theta} = sqrt{1 - cos^2{theta}} = sqrt{1 - frac{1}{3}} = sqrt{frac{2}{3}} = frac{sqrt{2}}{sqrt{3}}$$
Finally, the area of the triangle is:
$$
Area = frac{1}{2} (5 sqrt{3}) (15) frac{sqrt{2}}{sqrt{3}} = frac{75 sqrt{2}}{2}
$$
We can just multiply $frac{sqrt{2}}{sqrt{2}}$ to the area, and then we get the answer you posted:
$$
Area = frac{75 sqrt{2}}{2} left(frac{sqrt{2}}{sqrt{2}}right) = frac{75}{sqrt{2}}
$$
$endgroup$
add a comment |
$begingroup$
The correct formula is $mathbf{a} cdot mathbf{b} = begin{vmatrix} {mathbf{a}} end{vmatrix}begin{vmatrix} {mathbf{b}} end{vmatrix} cos{theta} $
So what you really have is $cos{theta} = cfrac{-1}{sqrt{3}}$
Therefore $$sin{theta} = sqrt{1 - cos^2{theta}} = sqrt{1 - frac{1}{3}} = sqrt{frac{2}{3}} = frac{sqrt{2}}{sqrt{3}}$$
Finally, the area of the triangle is:
$$
Area = frac{1}{2} (5 sqrt{3}) (15) frac{sqrt{2}}{sqrt{3}} = frac{75 sqrt{2}}{2}
$$
We can just multiply $frac{sqrt{2}}{sqrt{2}}$ to the area, and then we get the answer you posted:
$$
Area = frac{75 sqrt{2}}{2} left(frac{sqrt{2}}{sqrt{2}}right) = frac{75}{sqrt{2}}
$$
$endgroup$
The correct formula is $mathbf{a} cdot mathbf{b} = begin{vmatrix} {mathbf{a}} end{vmatrix}begin{vmatrix} {mathbf{b}} end{vmatrix} cos{theta} $
So what you really have is $cos{theta} = cfrac{-1}{sqrt{3}}$
Therefore $$sin{theta} = sqrt{1 - cos^2{theta}} = sqrt{1 - frac{1}{3}} = sqrt{frac{2}{3}} = frac{sqrt{2}}{sqrt{3}}$$
Finally, the area of the triangle is:
$$
Area = frac{1}{2} (5 sqrt{3}) (15) frac{sqrt{2}}{sqrt{3}} = frac{75 sqrt{2}}{2}
$$
We can just multiply $frac{sqrt{2}}{sqrt{2}}$ to the area, and then we get the answer you posted:
$$
Area = frac{75 sqrt{2}}{2} left(frac{sqrt{2}}{sqrt{2}}right) = frac{75}{sqrt{2}}
$$
answered Aug 14 '18 at 3:18
HugoTeixeiraHugoTeixeira
3281313
3281313
add a comment |
add a comment |
$begingroup$
Since your vectors are in $mathbb{R}^3$, you can find the area of the parallelogram generated by the vectors by computing the magnitude of the cross product. The area of the triangle is half that value:
$Area=(1/2) | a times b |$.
$endgroup$
$begingroup$
Alternative approach is en.wikipedia.org/wiki/Heron%27s_formula
$endgroup$
– user2661923
Dec 31 '18 at 14:23
add a comment |
$begingroup$
Since your vectors are in $mathbb{R}^3$, you can find the area of the parallelogram generated by the vectors by computing the magnitude of the cross product. The area of the triangle is half that value:
$Area=(1/2) | a times b |$.
$endgroup$
$begingroup$
Alternative approach is en.wikipedia.org/wiki/Heron%27s_formula
$endgroup$
– user2661923
Dec 31 '18 at 14:23
add a comment |
$begingroup$
Since your vectors are in $mathbb{R}^3$, you can find the area of the parallelogram generated by the vectors by computing the magnitude of the cross product. The area of the triangle is half that value:
$Area=(1/2) | a times b |$.
$endgroup$
Since your vectors are in $mathbb{R}^3$, you can find the area of the parallelogram generated by the vectors by computing the magnitude of the cross product. The area of the triangle is half that value:
$Area=(1/2) | a times b |$.
answered Aug 14 '18 at 3:40
NicNic8NicNic8
4,14531023
4,14531023
$begingroup$
Alternative approach is en.wikipedia.org/wiki/Heron%27s_formula
$endgroup$
– user2661923
Dec 31 '18 at 14:23
add a comment |
$begingroup$
Alternative approach is en.wikipedia.org/wiki/Heron%27s_formula
$endgroup$
– user2661923
Dec 31 '18 at 14:23
$begingroup$
Alternative approach is en.wikipedia.org/wiki/Heron%27s_formula
$endgroup$
– user2661923
Dec 31 '18 at 14:23
$begingroup$
Alternative approach is en.wikipedia.org/wiki/Heron%27s_formula
$endgroup$
– user2661923
Dec 31 '18 at 14:23
add a comment |
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1
$begingroup$
$mathbf{a}cdot mathbf{b}=|mathbf{a}||mathbf{b}| cos theta$.
$endgroup$
– Meow
Feb 7 '14 at 16:54
$begingroup$
Haha, thank you, can't believe I didn't check that.
$endgroup$
– Elise
Feb 7 '14 at 16:55
2
$begingroup$
Use the cross product ...
$endgroup$
– Mark Bennet
Feb 7 '14 at 17:31
$begingroup$
@MarkBennet that would make it a lot more efficient, thank you :)
$endgroup$
– Elise
Feb 7 '14 at 18:05