а) 7 / (x + y) = 7x / x * (x + y) = 7x / (x^2 - y);
б) 2 / (x - y) = 2 * (x - y) / [(x - y) * (x - y)] = 2 * (x - y) / (x - y)^2;
в) n / (m - n) = [ [n / (m - n)] * (a^2 - b^2) ] / (a^2 - b^2) = [n * (a^2 - b^2) / (m - n)] / (a^2 - b^2);
г) c / ( a - b) = [ [c * (m^3 - n^3)] / (a - b] / (m^3 - n^3).