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We have points A, B & C in 2D plane. How having point coordinates $(x, y)$ to calculate area of triangle formed by them?

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    English, please.2010-11-16

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To make Rahul's comment more explicit, the determinant formula

$\frac12 \begin{vmatrix}x_1&y_1&1\\x_2&y_2&1\\x_3&y_3&1\end{vmatrix}$

where the $(x_i,y_i)$ are the coordinates of the corners, gives the (signed) area of the triangle. For a guaranteed positive result, the points are to be taken anticlockwise.

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    @Rahul: Yeah, the couple of times I had to use both determinants and norms, I always used $|\det\mathbf{A}|$.2010-11-16
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If by square you mean the area, Heron's formula is your friend. Just calculate the side lengths and the semiperimeter.

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    @Rahul: Right you are.2010-11-16