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Math Help - Area of Triangle

  1. #1
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    Area of Triangle

    Hey,

    Im having trouble with these two questions, im getting stuck when im multiplying AB x AC

    Thanks
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  2. #2
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    Quote Originally Posted by mikel03 View Post
    Hey,

    Im having trouble with these two questions, im getting stuck when im multiplying AB x AC

    Thanks
    I'm not sure what you mean by multiplying.... do you mean the cross product?

    If so

    First we need to create two vectors that are sides of the triangle

    Using the head - tail method

    if A and B will the heads and C the tail so we get

    (1,2,3)-(3,4,5)=(-2,-2,-2) ad

    (-1,5,-1)-(3,4,5)=(-4,1,-6)

    Now we want half of the magnitude of the cross product and that will be the area of the triangle.

    \begin{vmatrix}<br />
\vec i & \vec j & \vec k\\<br />
-2 & -2 & -2 \\<br />
-4 & 1& -6 \\<br />
\end{vmatrix}=(12+2)\vec i-(12-8)\vec j+(-2-8)\vec k = 14 \vec i -4 \vec j -10 \vec k

    Now we just take half of the magnitude of this vector and we are done

    \frac{1}{2}\sqrt{14^2+(-4)^2+(-10)^2}=\frac{1}{2}\sqrt{196+16+100}=\frac{1}{2}\sq  rt{312}
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