Question

the sum of n terms of the series 4+6+9+13...........is
A) (n/6)(n²+3n+20) B)(n/6)(n²-3n-20)
c) (n²+3n+20) D) none

Answer 1

let's see if we find a pattern

sum(1) = 4
sum(2) = 4+6 = 10
sum(3) = 4 + 6 + 9 = 19
sum(4) = 19+13 = 32

using a) the corresponding answers would be
4 10 19 32 mmmmhhh , looking good
using b) the terms are -11/3 .... noway
using c) the terms are 24 ... forget it!

could it be just a coincidence?
try sum(5)
sum(5) = 4+6+9+13+18 = 50
using a)
sum(5) = (5/6)(25+15+20) = 50

I would say A)