Question

The sum of the first and 17th terms of an arithmetic sequence is 40.the sum of its fi$t and 18th terms is 43. Find the terms.what is the sum of its 7th and 11th terms?

Answer 1

let first term be a n 17th term be a +16d, 18th term be a+17d.......so from question
a+a+16d=40
2a+16d=40
similarly
2a+17d=43

hence 2a+16d+d=43
40+d=43
d=43-40=3
substitute this in 2a+16d=40
2a+ 16×3=40
2a=40-48
a=-8/2
a=-4

hence 7th term= a+6d
11th term= a+ 10d
adding these two gives 2a+16d=40