4、a³+2a²-12a+15 =a³+125+2a²-12a-110 =(a+5)(a²-5a+25)+2(a²-6a-55) =(a+5)(a²-5a+25)+2(a+5)(a-11) =(a+5)(a²-5a+25+2a-22) =(a+5)(a²-3a+3)
5、(x+y)^4+x^4+y^4=(x+y)^4+(x+y)(x^3-x^2y-xy^2+y^3)=(x+y)(2x^3+2x^2y+2xy^2+2y^3)= 2(x+y)[(x+y)x^2+(x+y)y^2]=2(x+y)^2(x^2+y^2)