Question

Find the sum of 10 terms of A.P. 9, 17, 25, ...

Hint:

Sum of A.P.: Use \( S_n = \frac{n}{2} [2a + (n-1)d] \).

Answer 1

Given A.P.:
\[ a = 9, \quad d = 17 - 9 = 8, \quad n = 10 \] Sum of first \( n \) terms:
\[ S_n = \frac{n}{2} [2a + (n-1)d] \] \[ S_{10} = \frac{10}{2} [2(9) + (10-1)(8)] \] \[ = 5 [18 + 72] = 5 \times 90 = 450 \] Thus, \( S_{10} = \mathbf{450} \).
Correct Answer: \( 450 \)