-4x2+25=25-4x^2=(5+2x)(5-2x)
（x+3y)(2x-y)=2x^2-xy+6xy-3y^2=2x^2+5xy-3y^2
2(x+5)(x-4)-3(x-6)(x+1)=2(x^2+x-20)-3(x^2-5x-6)=-x^2+17x-22
13.252-6.252=(13+0.252)-(6+0.252)=13+0.252-6-0.252=7
4x（a-b)-8y(b-a)=4x(a-b)+8y(a-b)=(a-b)(4x+8y)=4(a-b)(x+2y)