1、原式=1/4（4a²-b²）=1/4(2a+b)(2a-b)
2、原式=(3(2x-y))²-(2(x+2y))²
=(3(2x-y)+2(x+2y))(3(2x-y)-2(x+2y))
=(8x+y)(4x-7y)
3、x²-4y²=(x+2y)(x-2y)=3*(-2)=-6
4、（m+2n）²-（3m-n）²=((m+2n)+(3m-n))((m+2n)-(3m-n))
=-(4m+n)(2m-3n)
=- 90*10
=-900