A-Level Maths Edexcel 9MA0

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#1 Find sum of first n terms of arithmetic series

Use Sn=n2(a1+an)S_n = \frac{n}{2}(a_1 + a_n) or Sn=n2(2a1+(n−1)d)S_n = \frac{n}{2}(2a_1 + (n-1)d).