Delphis C2 sequences question
On this delphis paper (http://pmt.physicsandmathstutor.com/download/Maths/A-level/C2/Papers-Delphis/for-OCR/Delphis%2004%20-%20C2%20OCR.pdf) for question 2, I thought that I had to use the the 1/2 n {2a + (n-1)d} formula, but they have done it in a completely different way in the mark scheme (see final page of the pdf). I really don't understand what they have done - could someone please explain it to me.
Thanks
Edit: Actually, I think I've got it, but I just want to double check. Is it because it shows that r=0 and s=1 so s must be r +1?
Thanks
Edit: Actually, I think I've got it, but I just want to double check. Is it because it shows that r=0 and s=1 so s must be r +1?
(edited 6 years ago)
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