Объяснение:

5)(х+7)(7-х)=7х+49-х^2-7х=49-х^2

6)(5а-8b)(5a+8b)=25a^2-40ab+40ab-64b^2=

=25a^2-64b^2

7)(8m+2)(2-8m)=16m+4-64m^2+16m=

=32m+4-64m^2

8)(13c-14d)(14d+13c)=182cd-196d^2+169c^2-182cd=

=-196d^2+169c^2