Q17 of 89 Page 45

In an AP, if a = 1, a_n = 20 and S_n = 399, then n is equal to

Given

First term, a = 1,

Nth term, a_n = 20,

Some of n terms, S_n = 299

Using the formula

Where S_n = Sum of first n terms

n = no of terms

l = a_n = last term

2S_n = n(a + a_n)

2(399) = n(1 + 20)

n = 798/21

n = 38

If the first term of an AP is - 5 and the common difference is 2, then the sum of the first 6 terms is

The sum of first 16 terms of the AP 10, 6, 2, ….. is

The sum of first five multiples of 3 is

Which of the following form of an AP? Justify your answer.

–1, –1, –1, –1, ….

In an AP, if a = 1, an = 20 and Sn = 399, then n is equal to