a2-b2=(a-b)(a+b)
(9-x)*(9+x)=-
2
(a-9)*(a+9)=2-
(9-cd)*(9+cd)=-(
)2
(9b-9)*(9b+9)=*
2-
(9b-9)*(9b+9)=(*
)2-
(12-m)*(12+m)=-
2
(z-12)*(z+12)=2-
(12-cd)*(cd+12)=-(
)2
(9b-12)*(9b+12)=*
2-
(9b-12)*(12+9b)=(*
)2-
(9ab-zx)*(zx+9ab)=*(
)2-(
)2
(9z+a)*(a-9z)=2-
*
2
(mlu+9k)*(9k-mlu)=*
2-(
)2
(12bc3-9x)*(9x+12bc3)=*b2c6-
*
2
(12bc3-9x)*(9x+12bc3)=(*bc3)2-
*
2