Question

The first and last terms of an AP are 10 and 100. If the sum of all its terms is 400, then the number of terms will be??

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Answer 1

a1 = 10

an = 100

sn = n/2(a1 + an)

400 = n/2(10+100)

400*2 = n(110)       =      800/110 = n

n = 80/11

n = 7.27