因式分解(加50分）

4x^n-13x^2y^2+9y^4

=(2x-3y)^2

xy^3+y^2+y-x+1
= x(y^3-1)+y^2+y+1
= x(y-1)(y^2+y+1)+y^2+y+1
=(xy-x+1)(y^2+y+1)

a^2+2ab+2ac-2bc-3b^2
= a^2+2ab-3b^2+2ac-2bc
= (a+3b)(a-b) +2c(a-b)
=(a-b)(a+3b+2c)

(a-1)(a-3)(a-6)(a+2)+56 [常数项=-20+56=30=5*2*-3*-1]
= （a+1)(a+3)(a-2)(a-5)[之所以不取，3，1，是因为，原式有，+56不能]