17/12x8/13-5/13x8/12=8/12x17/13-5/13x8/12=8/12x(17/13-5/13)=8/12x12/13=8/13
(17/12)*(8/13)-(5/13)*(8/12)=(17/12)*(8/13)-(5/12)*(8/13)=(17/12-5/12)*(8/13)=8/13
