令PAB的高(AB边上)为X,AB的一半为Y,角PAB为M,角PCB为N, N+2M=PI/2。X也是PAC的(AC边上)的高。X/3=SIN(N)=COS(2M)=COS(M)^2 - SIN(M)^2=Y^2/3 - X^2/3.即X = Y^2 - X^2,再加上3 = Y^2 + X^2.相减即得2X^2 = 3 - X.