1/n(n + 2) = 1/2*(n + 2 - n)/n(n + 2) = 1/2*(1/n - 1/(n + 2))
1/3 + 1/15 + 1/35 + 1/65 + 1/99 = 1/(1*3) + 1/(3*5) + 1/(5*7) + 1/(7*9) + 1/(9*11) = 1/2*(1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11) = 1/2*(1 - 1/11) = 5/11
1/n(n + 2) = 1/2*(n + 2 - n)/n(n + 2) = 1/2*(1/n - 1/(n + 2))
1/3 + 1/15 + 1/35 + 1/65 + 1/99 = 1/(1*3) + 1/(3*5) + 1/(5*7) + 1/(7*9) + 1/(9*11) = 1/2*(1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11) = 1/2*(1 - 1/11) = 5/11