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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To make Rahul's comment more explicit, the determinant formula
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.
If by square you mean the area, Heron's formula is your friend. Just calculate the side lengths and the semiperimeter.