Find the sum of the series:
I do the sum and each time I get a different answers, none of them matching with the answer in my book.
My latest answer: -3n(2n-1)
My latest working:
starting from r=1, there is (n-1) terms and 1^2 have to be added separately.
starting from r=1, there is n terms.
Can you please narrow down the field where I should search the error?
I am exasperated looking through my rough handwriting.