Question

Find the number of terms, n, of the arithmetic series given a = 21, an = 39, and Sn = 210.

A.7
B. 8
C. 9
D.5

Answer 1

The number of terms n is 7.

Step-by-step explanation:

Given that:

First term a=21

nth term
`a_(n)`=39

Sum up to n terms
`S_(n)`=210

To Find:

The number of terms n in the arithmetic progression.

Formula to be used:

Sum up to n terms Sn is given by the expression

`S_(n) =(n)/(2) (a+a_(n) )`

Solution:

`S_(n) =(n)/(2) (a+a_(n) )\\\\210=(n)/(2)(21+39)\\\\210=(n)/(2)(60)=30n\\\\n=210/30=7`

Number of terms n in the given series is n=7