This is the basic and conceptual question from Chapter name- Arithmetic Progression

Chapter number- 9

Exercise 9.6

In this question we have been given that nth term of an A.P is given by (−4n + 15).

Now we have to Find the sum of first 20 terms of this A.P

CBSE DHANPAT RAI publications

Class:- 10th

Solutions of CBSE Mathematics

Question 32

We are given the nth term of the A.P., a

_{n}= −4n + 15.On putting n = 1 to find the first term of our A.P., a = −4(1) + 15 = 11

On putting n = 20 to find the 20th term of the A.P., a20 = −4(20) + 15 = −65

By using the formula of the sum of first n terms of an A.P.

S

_{n}= n[a + a_{n}] / 2.So, S

_{20 }= 20[11 + (−65)] / 2= 10(−54) = −540

Hence, the sum of first 20 terms of this A.P. is −540.