Question
Fri November 04, 2011 By:
 

If 7times the 7th term of an A.P is equal to 11 times the 11th term,prove that 18th term is equal to zero.

Expert Reply
Fri November 04, 2011

let a = first term and d=common difference.

According to question,

7a7 = 11 a11

=>7(a+6d) = 11(a+10d)

=> 7a-11a = 110d-42d

=> -4a = 68d

=>a = -17d  ......(1)

Now , a18 = a +17d 

but a = -17 d  ..by (1)

so put this value , we get,

a18=-17d +17d =0 

Hence proved.

Home Work Help