Question

Find the sum of the series 1^2 ,3^2 ,5^2 .....to n terms

Answer 1

 I took two terms of the series as (2k-1)2 - (2k)2 = 1-4k

So adding the series as ∑(1-4k) taking range of k from 1 to n/2 (since I'm taking two terms at the same time), I got the answer :- -n(n+1)/2
but my textbook has given the answer as :- -n(2n+1)