20.1.xy(4x2+4xy+y2)=xy(2x+y)2
20.2 x(9x2-25y2)=x(3x-5y)(3x+5y)
20.3 -3x(x2-2x+1)=-3x(x-1)2
21 [2a2+1/2b2](2a2-1/2b2)=4a4-1/4b4 =4*1-1/4*16=0
22 (x-y)2=x2-2xy+y2=x2+2xy+y2-4xy=(x+y)2-4xy=16-8=8
x3+y3=(x+y)(x2-xy+y2)=(x+y)(x2+2xy+y2-3xy)=(x+y)[(x+y)2-3xy]=4*[16-3*2]=40