(1-(a^-n+b^-n)/(a^-n-b^-n))^-2

(1 - (a^(-n) + b^(-n))/(a^(-n) - b^(-n)))^(-2) = ((a^(-n) - b^(-n) - a^(-n) - b^(-n))/(a^(-n) - b^(-n)))^(-2) = (-2•b^(-n)/(a^(-n) - b^(-n)))^(-2) = ((-2/b^n)/(1/a^n - 1/b^n))^(-2) = ((-2/b^n)/((b^n - a^n)/(a^n•b^n)))^(-2) = (-2/((b^n - a^n)/a^n))^(-2) = (-2•a^n/(b^n - a^n))^(-2) = (b^n - a^n))^2/(4•a^(2n))