正弦定理，AC^2 = 4 + y^2 = x^2 + 1, 余弦定理，BD^2 = 1+y^2 + y = x^2 + 4 -x,
解，y^2 + 1 = x^2 - 2, y - 2 = 4-x, x+y = 6, x = 13/4, y = 21/4, 面积 = 1/2 （2*y + x) = 55/8
如图，c^2 = 2ab + (b-a)^2 = a^2 + b^2
