Question

how many terms of ap 9,17,25,..….... would give the sum of 636

Answer 1

a=9
c.d=8
sn=636
n/2{2a+(n-1)d}=sn
n/2{2(9)+(n-1)(8)}=636
n/2{18+8n-8}=636
n{10+8n}=1276
10n+8n sq. =1276
8n sq +10n=1276
2(4n sq+5n) =1276
4n sq +5n =636
4n sq + 5n -636=0
4n sq +(53-48)n -636=0
4n sq + 53n -48n -636= 0
n(4n+53) - 12(4n+53)=0
(n-12) (4n+53)=0
n=12 n=-53/4
n can not be negative
n=12