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Mary Stephen
Answers (1)
I think this method is for a. P, not gp The correct one is ar=-6 ar^4=48 r^3=-8 So r=-2 Sn=a(1-r^n) - - - - - - - - 1-r S4=3(1-(-2^4) - - - - - - - - 1-(-2) S4=3(1-16) - - - - - 1+2 S4=3(-15) - - - - - 3 S4=-15 That's the correct answer.
