Question

(ii) If a = -2, d= -5, an= -77 find n and sn.​

Answer 1

The answers are n = 16 ans S$_{n}$ = - 632  

Given: First term a = -2  and common difference d = -5  

nth term a$_{n}$ = - 77

To find: The value of n and S$_{n}$

Solution:

As we know nth term in Ap = a+(n-1)d

a$_{n}$ = - 77  ⇒  - 2 +(n-1) (-5) = - 77

⇒ -2 - 5n + 5 = -77        

⇒  - 5n + 5 = -75

⇒  - 5n = - 77 - 5

⇒ - 5n = - 80   ⇒  n = 16  

Here, S$_{n}$ = sum of n term in Ap

As we know sum of n term S$_{n}$ = n/2 (first term + last term)  

= $\frac{16}{2} [-2 +(-77) ]$ = 8 [ -79 ] = - 632  

Therefore,  n = 16 ans S$_{n}$ = - 632  

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