x-xy2=x(1-y2)=x(1+y)(1-y)
3xy3-3xy=3xy(y2-1)=3xy(y+1)(y-1)
-36x2+49/64y2=(7/8y)2-(6x)2=(7/8y+6x)(7/8y-6x)
0.2a2-9/20b2=1/5(a2-9/4b2)=1/5(a+3/2b)(a-3/2b)
5m2a4-5m2b4=5m2(a4-b4)= 5m2(a2+b2)(a2-b2)= 5m2(a2+b2)(a+b)(a-b)
16x2y2z2-9=(4xyz)2-9=(4xyz+3)(4xyz-3)
81(a+b)2-4(a-b)2=[9(a+b)+2(a-b)][9(a+b)-2(a-b)]=(9a+9b+2a-2b)(9a+9b-2a+2b)=(11a+7b)(7a+11b)
(2x+3y)2-(3x-2y)2=(2x+3y+3x-2y)(2x+3y-3x+2y)=(5x+y)(5y-x)
9(m+n)2-(m-n)2=[3(m+n)+(m-n)][3(m+n)-(m-n)]=(3m+3n+m-n)(3m+3n-m+n)=(4m+2n)(2m+4n)=4(2m+n)(m+2n)
(x-y)2/4-(x+y)2/4=[(x-y)/2+(x+y)/2][(x-y)/2-(x+y)/2]=-xy