Question

find S15 in the A.P =5,0,-5,-10.​

Answer 1

S15 in the A.P =5,0,-5,-10.

a=t1=5

t2=0

d=t2 - t1

=0-5= -5

n=15

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

S15= 15/2 [2×5+(14)(-5)]

S15=15/2 [2×5 -70]

S15=15/2[-60]

S15=15×(-30)

S15=(-450)

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Answer 2

a=5 d=-5 n=15

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

S15= 15/2[2×5+(15-1)-5]

S15= 15/2[10+(-70)]

S15= 15/2(-60)

S15= -450

Ans= -450

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I hope it is helpful for you.