FP1 resit tomorrow, i have no hope and can't even do this one.... theres no identity for sigma 1/r.
its exercise 2b, question 8, in old fp1.
sigma to 20 of 1 over (r+3)(r+6)
i would draw it out nicely for you guys but i have no idea how.
answer is 0.1655
a worked answer would be nice.