Posted by **lana** on Wednesday, July 25, 2007 at 2:18pm.

ABCD is a square . if the coordinates of three of its vertices are A(-1,2a), B(a,2a), C(a,0), find the coordinates of D.

BC is obviously 2a (it's a vertical line)

then AB must be 2a (notice AB is horizontal)

so BC = AC because it is a square

2a = a+1

a=1

Now you can find the coordinates of A, B, and C, it should be easy for you to find D.

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