Question

how many terms of the ap : 9, 17 ,25,........ must be taken to give a sum of 636?

Answer 1

$\frac{n}{2} (2 \times 9 + (n - 1) \times 8) = 636$
Now solve for n... And get your answer

Answer 2

a=9
d=8
Sn=636
Sn=n/2[2a+(n-1)d]
636=n/2[2×9+(n-1)8
636=n/2[18+8n-8]
636=n/2[10+8n]
636=5n+4n^
4n^+5n-636=0
Then by quadratic formula solve this equation and get n.