1.-a+2a²-a³
=-a（1-2a+a²)
=-a（1-a)²
2.9(x+2y)²-25(x-2y)²
=［ 3(x+2y) ］²-［5(x-2y) ］²
=[3(x+2y)+5(x-2y)][3(x+2y)-5(x-2y)]
=(8x-4y)(-2x+16y)
=-8(2x-y)(x-8y)
3.x²+x+_1/4__=( x+1/2)²

4.(7a-8b)²-4x²
=(7a-8b)²-（2x)²
=(7a-8b+2x)(7a-8b-2x)

5.9(a-b)²+12(a-b)+4
=[3(a-b)]²+2×3(a-b)×2+2²
=[3(a-b)+2]²
=(3a-3b+2)²