I'm very confused on how to solve this? i tried solving it the normal way i would if r=1, except by multiplying by 3 first. but the markscheme said it was wrong. (the answer should be 2044) how do i solve this? please explain.
Write out the first few terms to get the initial term and the difference term, then use the sum of an arithmetical series, which you can derive easily:
Write out the first few terms to get the initial term and the difference term, then use the sum of an arithmetical series, which you can derive easily:
I'm very confused on how to solve this? i tried solving it the normal way i would if r=1, except by multiplying by 3 first. but the markscheme said it was wrong. (the answer should be 2044) how do i solve this? please explain.
∑r=330=∑r=130−∑r=12
because you'd have (u1+u2+u3+u4+⋯+u3)−(u1+u2)