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.