Question

obtain the sum of first fifteen term of an A.P. whose first term is 10 and common difference is -7
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Answer 1

Answer:885

Step-by-step-explanation:

a=10

d=-7

n=15

Sn=n/2[2a+(n-1)d]

S15=15/2[2×10+(15-1)-7]

S15=15/2[20+14×7]

S15=15/2[118]

S15=7.5(118)

S15=885