Помогите срочно 7x−1−y−17x−1+y−1, если yx=8−1.

Решите задачу:

$\frac{7x^{-1}-y^{-1}}{7x^{-1}+y^{-1}}=\frac{\frac{7}{x}-\frac{1}{y}}{\frac{7}{x}+\frac{1}{y}}=\frac{\frac{7y-x}{xy}}{\frac{7y+x}{xy}}=\frac{(7y-x)*xy}{(7y+x)*xy} =\frac{7y-x}{7y+x}=\frac{\frac{7y}{x}-\frac{x}{x}}{\frac{7y}{x}+\frac{x}{x}}=\frac{7*\frac{y}{x}-1}{7*\frac{y}{x}+1}} =\frac{7*8^{-1}-1 }{7*8^{-1}+1 }=\\=\frac{7*\frac{1}{8}-1 }{7*\frac{1}{8}+1} =\frac{\frac{7}{8}-1 }{\frac{7}{8}+1 }=-\frac{1}{8}*\frac{8}{15} =\frac{-1}{15}$