Question

9. an AP of 21 terms, the sum of the first 3
terms is -
33 and that of the middle 3 is
75 What is the sum of the AP?​

Answer 1

Answer:

The sum of the first 3 terms is (3a + 3d) = -33 and the sum of the middle 3 terms is (3a + 30d) = 75. Solving these two equations, we get a = -15 and d = 4. The sum of the 1st 21 terms is (21/2) (2 * -15 + 20 * 4) = 525