Q14 of 102 Page 410

How many terms of the AP 26, 21 16, 11, …. are needed to give the sum 11?

To Find: Number of terms required

Let the number of terms be n.

⇒n[52 - 5n + 5] = 22
⇒n(57 - 5n) = 11 × 2 = 11[57 - 5(11)]
⇒n = 11

11 terms are required to give the sum 11.

Find the sum of n term of an AP whose r^th term is (5r + 1).

If the sum of a certain number of terms of the AP 27, 24, 21, 18, …. is –30, find the last term.

How many terms of the AP 18 16, 14, 12, …. are needed to give the sum 78? Explain the double answer.

How many terms of the AP 20, must be taken to make the sum 300? Explain the double answer.