Question

The sum of the 3rd and 7th terms of an A.P. is 38, and the 9th term is 37. Find the A.P?​

Answer 1

Let a be the first term in the arithmetic progression. Then each successive term differs from a by a fixed number c, so that

• first term = a

• second term = a + c

• third term = (a + c) + c = a + 2c

• fourth term = (a + 2c) + c = a + 3c

and so on. In general, the n-th term in the AP is a + (n - 1) c.

The sum of the 3rd and 7th terms is 38, so that

(a + 2c) + (a + 6c) = 38

==>   2a + 8c = 38

==>   a + 4c = 19 … … … [1]

The 9th term is 37, so

a + 8c = 37 … … … [2]

Subtracting [1] from [2] eliminates a and lets you solve for c :

(a + 8c) - (a + 4c) = 37 - 19

4c = 18

c = 18/4 = 9/2

Solve for a using either equations [1] or [2] :

a + 8 (9/2) = 37

a + 36 = 37

a = 1

Then the n-th term in the AP is 1 + 9/2 (n - 1) or 9/2 n - 7/2, where n ≥ 1.