Arithmetic Series

Find n for the arithmetic series with Sn = 360, a1=42 and an= -26

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  1. Sn = n/2 (a1 + an)
    360 = n/2(42 - 26)
    360 = 8n
    n = 45

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  2. May I ask how did you get that 8n...I'm having trouble working it out on paper for another problem

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