Question

In an Ap, a=1, an=20, sn=399 then find n

Answer 1

Answer:

Sn=n/2(a+l)

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399=n/2(1+20)

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399*2/21=n

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19*2=n

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N=38

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Hope this helps you

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Answer 2

Answer:

sn=n/2(a+an)

399=n/2(1+20)

798=21n

798/2=n

38=n

so there are 38 terms

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