Math, asked by kanishpoojari31, 4 days ago

The first term of an AP is 1 and the nth term is 20.If Sn=399,what is n ​

Answers

Answered by vbhogal5
0

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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