a2-b2=(a-b)(a+b)
(7-x)*(7+x)=-
2
(a-7)*(a+7)=2-
(7-cd)*(7+cd)=-(
)2
(9b-7)*(9b+7)=*
2-
(9b-7)*(9b+7)=(*
)2-
(1-m)*(1+m)=-
2
(z-1)*(z+1)=2-
(1-cd)*(cd+1)=-(
)2
(7b-1)*(7b+1)=*
2-
(7b-1)*(1+7b)=(*
)2-
(9ab-zx)*(zx+9ab)=*(
)2-(
)2
(9z+a)*(a-9z)=2-
*
2
(mlu+9k)*(9k-mlu)=*
2-(
)2
(1bc3-9x)*(9x+1bc3)=*b2c6-
*
2
(1bc3-9x)*(9x+1bc3)=(*bc3)2-
*
2