Q16 of 178 Page 548

Which term of the AP 21, 18, 15, ... is zero?

Here first term = 21

Common difference = 18 - 21 = - 3

Let a_n be the term which is zero.

∴ a_n = 0

⇒ a + (n - 1)d = 0

⇒ 21 + (n - 1)(-3) = 0

⇒ 21 - 3n + 3 = 0

⇒ 3n = 24

⇒ n = 8

∴ 8^th term of the given AP will be zero.

Write the next term of the AP

Find the sum of first n natural numbers.

Find the sum of first n even natural numbers.