Question

已知：M=2x的平方-5xy+6y的平方，N=7y的平方+4xy+4x的平方，求M+N的值

一变：求M-N的值
二变：求3M-2N的值

Answer 1

M+N
=2x^2-5xy+6y^2+7y^2+4xy+4x^2
=6x^2-xy+13y^2

M-N
=2x^2-5xy+6y^2-(7y^2+4xy+4x^2)
=2x^2-5xy+6y^2-7y^2-4xy-4x^2
=-2x^2-9xy-y^2

3M-2N
=3(2x^2-5xy+6y^2)-2(7y^2+4xy+4x^2)
=6x^2-15xy+18y^2-14y^2-8xy-8x^2
=-2x^2-23xy+4y^2