(1/a+1/b)^2/(1/a^2-1/b^2)=(1/a+1/b)^2/(1/a-1/b)(1/a+1/b)=(1/a+1/b)/(1/a-1/b)=ab*(1/a+1/b)/(ab*(1/a-1/b))=(b+a)/(b-a)
