Question

How many terms of the AP: 9, 17, 25, ... must be taken to give a sum of 636?
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Answer 1

Answer:

Explanation:

Here,

a  1 =9

a  2 =a  1 +d=17

9+d=17

d=8

 

We know that, the sum of n terms of an A.P. is,

sn=n/2[2a+(n-1)d]

According to the question,

636=  n/2 {2(9)+(n−1)8}

1272=18n+8nsquare−8n

8n  square+10n−1272=0

4n square+5n−636=0

n=12,−13.25

 

Ignore the negative value.

n=12

 

Hence, 12 terms of the A.P. is required to give a sum of 636.