十字相乘法,

1.9(a-b)²+12(a²-b²)+4(a+b)²
=[3(a-b)]²+2[3(a-b)][2(a+b)]+[2(a+b)]²
=[3(a-b)+2(a+b)]²
=(5a-b)²
2.(x-y)²-3(x-y)-40
1 5
1 -8
=[(x-y)+5][(x-y)-8]
=(x-y+5)(x-y-8)
3.(2x+y)²+(2x+y)-2
1 2
1 -1
=(2x+y+2)(2x+y-1)
4.x²+11xy+18y²
1 2
1 9
=(x+2y)(x+9y)
3.6x²-17x-10=(2x+1 )(3x-10)
3 -10
2 1