1、5x^2+6xy-8y^2
=(x+2y)(5x-4y)
2、4m^2+8mn+3n^2
=(2m+n)(2m+3n)
3、7（x-1)^2+4(x-1)(y+2)-20(y+2)^2
=[(x-1)+2(y+2)][7(x-1)-10(y+2)]
=(x+2y+3)(7x-10y-27)