a2-b2=(a-b)(a+b)
(2-x)*(2+x)=-
2
(a-2)*(a+2)=2-
(2-cd)*(2+cd)=-(
)2
(7b-2)*(7b+2)=*
2-
(7b-2)*(7b+2)=(*
)2-
(13-m)*(13+m)=-
2
(z-13)*(z+13)=2-
(13-cd)*(cd+13)=-(
)2
(2b-13)*(2b+13)=*
2-
(2b-13)*(13+2b)=(*
)2-
(7ab-zx)*(zx+7ab)=*(
)2-(
)2
(7z+a)*(a-7z)=2-
*
2
(mlu+7k)*(7k-mlu)=*
2-(
)2
(13bc3-7x)*(7x+13bc3)=*b2c6-
*
2
(13bc3-7x)*(7x+13bc3)=(*bc3)2-
*
2