二元二次方程(x+y)(x-2y)=-5,(x-y)(x+2y)=7 怎么解
问题描述：
二元二次方程(x+y)(x-2y)=-5,(x-y)(x+2y)=7 怎么解
答
(x+y)(x-2y)=(x+y)(x-y-y)=(x²-y²)-y(x+y)所以(x²-y²)-y(x+y)=-5 ①(x-y)(x+2y)=(x-y)(x+y+y)=(x²-y²)+y(x-y)所以(x²-y²)+y(x-y)=7 ②①-②得到xy=6,x²y²=36 ③...