def computePoint(triangle):
	"""
	Computes the last point D in an ABC square triangle 
	where a and c are the diagonal - triangle = [a,b,c]
	            D--- C
	            |  / |
	            | /  |
	            |/   |
	            A----B
	"""
	# get coordinates of each point a, b, c
	a, b, c = triangle
	xa, ya = a
	xb, yb = b
	xc, yc = c
	# due to subtriangle congruence
	xd = xc - (xb - xa)
	yd = (ya -yb) + yc
	d = (xd, yd)
	return d