Ответы и объяснения

2013-08-22T06:39:19+00:00
(5x-7y)(5x+7y)+(7x-5y)(7x+5y) =25 x^{2} - 49y^{2} +49 x^{2} -25y^2=
=74x^2 - 74y^2

2013-08-22T09:28:32+00:00
(5x-7y)(5x+7y)+(7x-5y)(7x+5y)=25x^2-49y^2+49x^2-25y^2=(25x^2-25y^2)+(49x^2-49y^2)=25(x^2-y^2)+49(x^2-y^2)=(25+49)(x^2-y^2)=74x^2-74y^2