簡單數學，唔明點解

(x+y-1-xy)^2
=(x-xy+y-1)^2
=[(x-xy)+y-1]^2
=[x(1-y)+y-1]^2
=[x(1-y)-1+y]^2
=[x(1-y)-(1-y)]^2
=[(1-y)(x-1)]^2
=(1-y)^2(x-1)^2
因為而家係^2, (1-y)^2=(y-1)^2
所以
(1-y)^2(x-1)^2
=(x-1)^2(y-1)^2

(x+y-1-xy)2
=(x-1-xy+y)2
=[(x-1)(1-y)]2
=(x-1)2[(-1)(y-1)]2
=(x-1)2(y-1)2