1.(2m^2-n)^2(2m^2+n)^2
=[(2m^2-n)(2m^2+n)]^2
=(4m^4-n^2)^2
=16m^8-8m^4n^2+n^4
2.(x-3y)(x+3y)-(x-3y)^2
=(x-3y)[(x+3y)-(x-3y)]
=(x-3y)6y
=6xy-18y^2
3.(x^2+nx+3)(x^2-3x+m)
=x^4-(3-n)x^3+(m-3n+3)x^2+mnx+3m
当m=6
n=3
就能把x^2和x^3项消去了
应该能看懂吧