x+y = a
x-y = b
所以
2x = a+b
2y = a-b
x= (a+b)/2
y = (a-b)/2
xy = 1/4 (a+b) (a-b) = (a^2- b^2)/4
或者这么解
(x+y)^2 = a^2
(x-y)^2 = b^2
(x+y)^2 -(x-y)^2 = 4xy = a^2 -b^2
xy =(a^2- b^2)/4