а²+d² a-d
_____ _ ______
a²-d² a+d

(a²+d²)/(a²-d²) - (a-d)/(a+d)=((a²+d²) - (a-d)(a-d))/((a-d)(a+d))=(a²+d² - (a-d)²)/((a-d)(a+d))=(a²+d²-a²+2ad-d²)/((a-d)(a+d)=2ad/((a-d)(a+d)=2ad/(a²-d²)