（X-1)^2 +4(X-1)(Y+2)-20(Y+2)^2
十字相乘法(X-1)(2-2倍根号6)(y+2)
(X-1)(2+2倍根号6)(y+2)
=[(X-1)+ (2-2倍根号6)(y+2)]*[(X-1)+ (2+2倍根号6)(y+2)]

(2+2倍根号6)(y+2)
(2-2倍根号6)(y+2)的得出是根据下面的方法得到的.
设两个未知数a ; b
a+b=4
a*b=20
联立成方程求出a,b 即可.