The first term of an AP is 1 and the nth term is 20.If Sn=399,what is n
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Answer:
Given : The first term of an A.P. is 1 and the nth term is 20. If Sn = 399
To find : The value of n ?
Solution :
The sum of n terms of A.P is given by,
S_n=\frac{n}{2}[a+l]S
n
=
2
n
[a+l]
Where, a is the first term a=1
l is the nth term l=20
S_nS
n
=399 is the sum of n terms
Substitute the value in the formula,
399=\frac{n}{2}[1+20]399=
2
n
[1+20]
798=21n798=21n
n=\frac{798}{21}n=
21
798
n=38n=38
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