13) c²-6c+9-4x²=(c-3)²-(2x)²=(c-3-2x)(c-3+2x);
14)4c²+20c+25-9a²=(4c²+20c+25)-(3a)²=(2c+5)²-(3a)²=
=(2c+5-3a)(2c+5+3a);
15)xy²+y+x²y+x+4xy+4=(xy²+x²y+4xy)+(y+x+4)=xy(y+x+4)+(y+x+4)=
=(y+x+4)(xy+1);
16)3x²+2x-xy-2y²+y³-3xy²=(3x²-3xy²)+(2x-2y²)-(xy-y³)=
=3x(x-y²)+2(x-y²)-y(x-y²)=(x-y²)(3x+2-y).
17)x²+x-xy-y²+y³-xy²=(x²-xy²)+(x-y²)-(xy-y³)=x(x-y²)+(x-y²)-y(x-y²)=
=(x-y²)(x+1-y);
18) a²x+a+ax²+x+2ax+2=a(a+x)+x(a+x)+2(a+x)=(a+x)(a+x+2).