it the nth of a sequence is denoted by the formula n(2n+1)-3n find the sum of d first four terms

Answer 1

2 n^2 + n -3n = 2 n^2 - 2 n = 2 n(n-1)
n = 1 ---> 0
n = 2 ---> 4
n = 3 ---> 12
n = 4 ---> 24
sum = 16+24 = 40