1/2+1/6+1/12+1/20+...1/72
=1-1/2 + 1/2-1/3 + 1/3-1/4 + ...+ 1/8 - 1/9
=1-1/9 = 8/9
∴1/(1/2+1/6+1/12+1/20+...1/72)=9/8