a2-b2=(a-b)(a+b)
(9-x)*(9+x)=-2
(a-9)*(a+9)=2-
(9-cd)*(9+cd)=-()2
(2b-9)*(2b+9)=*2-
(2b-9)*(2b+9)=(*)2-
(10-m)*(10+m)=-2
(z-10)*(z+10)=2-
(10-cd)*(cd+10)=-()2
(9b-10)*(9b+10)=*2-
(9b-10)*(10+9b)=(*)2-
(2ab-zx)*(zx+2ab)=*()2-()2
(2z+a)*(a-2z)=2-*2
(mlu+2k)*(2k-mlu)=*2-()2
(10bc3-2x)*(2x+10bc3)=*b2c6-*2
(10bc3-2x)*(2x+10bc3)=(*bc3)2-*2