Q9 of 38 Page 5


Find the sum of the series 51 + 50 + 49 + ..... + 21.

Given: 51 + 50 + 49 + ..... + 21

Clearly,
a = 51, d = - 1, T n = 21

∴ a + (n – 1) d = T n
⇒ 51 + (n – 1) (- 1) = 21
⇒ (n – 1) (- 1) = 21 – 51
⇒ n – 1 = 30
⇒ n = 31

⇒ Sum of the series = 1116

∴ The sum of the series 51 + 50 + 49 + ..... + 21 = 1116.

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