CBSE Class 10 Answered

The AP is 9, 17, 25,.......

a (first term) = 9

d (common difference) = 17-9 = 8

Let n be the required number of terms.

S_n (sum of the n terms) = 450

We know that S_n = n/2 (2a + (n-1)d )

450 = n/2 (18 + (n-1)8 )

900 = n(18 + 8n -8)

900 = 8n² + 10n

8n² + 10n -900 = 0

8n² +80n -90n -900 = 0 

8n (n +10) -90 (n- 10) = 0

(n-10)(8n-90) = 0

n = 10 or 11.25

But n has to be a natural, since number of terms is always a natural number.

Thus, the required number of terms is 10.