Упростим:

((5 * x + 7 * y)^2 - 25 * x^2 - 49 * y^2) : (10 * x * y);

(25 * x^2 + 2 * 5 * 7 * x * y + 49 * y^2 - 25 * x^2 - 49 * x^2)/(10 * x * y);

10 * 7 * x * y/(10 * x * y) = 7.