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Question

Question 17
In an AP, if a = 1,
an=20
and Sn = 399,
then n is equal to :


A) 19
B) 21
C) 38
D) 42

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Solution

The sum up to n terms of an A.P is given by,
Sn=n2[2a+(n1)d]
399=n2[2×1+(n1)d]
798=2n+n(n1)d ...(i)
and an=20
a+(n1)d=20 ...(ii) [an=a+(n1)d]
Using eq.(ii) in eq.(i), we get,
798 = 2n + 19n
798 = 21n
n=79821=38

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