English

The sum of first 15 terms of an A.P. is 750 and its first term is 15. Find its 20th term.

Advertisements
Advertisements

Question

The sum of first 15 terms of an A.P. is 750 and its first term is 15. Find its 20th term.

Sum
Advertisements

Solution

Let a be the first term and d be the common difference.

Now, a = 15

Sum of first n terms of an AP is given by,

Sn = `"n"/(2)[2"a" + ("n" - 1)"d"]`

⇒ S15 = `(15)/(2)[2"a" + (15 - 1)"d"]`

⇒ 750 = `(15)/(2)(2"a" + 14"d")`

⇒ a + 7d = 50

⇒ 15 + 7d = 50

⇒ 7d = 35

⇒ d = 5

Now, 20th term = a20

= a + 19d

= 15 + 19 × 5

= 15 + 95

= 110.

shaalaa.com
  Is there an error in this question or solution?
Chapter 9: Arithmetic and Geometric Progressions - Exercise 9.3

APPEARS IN

ML Aggarwal Understanding Mathematics [English] Class 10 ICSE
Chapter 9 Arithmetic and Geometric Progressions
Exercise 9.3 | Q 5.2
Share
Notifications

Englishहिंदीमराठी


      Forgot password?
Use app×