Need help on c2 question arithmetic

You need to use the equation Sn =1/2n[2a+(n−1)d]
You know from part a and b that a=87 and d=-3 and you are told that n=k and Sn=0 so sub these numbers in and solve to find k

Is it right so far?, I feel like am doing it wrong


Posted from TSR Mobile

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Not quite - on the second line of that question you've done 87 +2 not x2, and it should be -3k not +3k

Like this? It still feels wrong lol


Posted from TSR Mobile

You don't need to expand the brackets. As the two things multiplied together make 0, one of them must equal 0. 1/2k can't = 0 as that would make k 0 and that wouldn't work with what you were given (you can't add up the first 0 terms) therefore 177-3k=0 and then solve that to give k=59