Question

The sum of the 5th and 9th terms of an A.P is 26 and the sum of its 7th and 1 1th terms is
42. Find the first three terms of the A.P

Answer 1

Step-by-step explanation:

The sum of the 5th and 9th terms of an A.P is 26 and the sum of its 7th and 1 1th terms is

42. Find the first three terms of the A.P

Answer 2

Answer:

Given an AP: Fifth term, a5 =26

Nineth term, a9 =42

To find:

Sum of first 15 terms of the AP nth term of AP is given by: an =a+(n−1)da5 =26=a+(5−1)d...............(i)

a 9=42=a+(9−1)d...............(ii)

Subtracting (i) from (ii), we get

16=4d

d=4

Substituting value of d in (i), we get

26=a+4×4

a=10

Now, sum of first n terms is given by:

S n = 2n

(2a+(n−1)d)

Substituting values, we get sum of fifteen terms as S 15= 215

(2×10+(15−1)×4)

S 15

=570