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., an = −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.
Sn = n[a + an] / 2.
So, S20 = 20[11 + (−65)] / 2
= 10(−54) = −540
Hence, the sum of first 20 terms of this A.P. is −540.