Question

The first and the last terms of an A.P. are 8 and 350 respectively. If its common difference is 9, how many terms are there and what is their sum?

Answer 1

First term, a = 8 

Common difference, d = 9 

Let the n^th term be the last term. 

∴ l = a_n = 350 

⇒ a + (n − 1) d = 350 

⇒ 8 + (n − 1) × 9 = 350 

⇒ (n − 1) × 9 = 342 

`rArr n-1=342/9=38`

`rArrn=38+1=39`

Thus, there are 39 terms in the given A.P. 

Sum of 39 trems , `S_39=39/2(a+a_39)`

`=39/2xx(8+350)`

`=39/2xx358`

`=6981`