Q) The A.P 8, 10, 12,……. has 60 terms. Find the sum of last 10 terms.

[Q 21A – Sample Question Paper – CBSE Board 2026]

Ans:

Step 1: Let’s start with first term and common difference of the given AP:

First term, a = 8 and the common difference, d = 10 – 8 = 2

Step 2: Next, Sum of last 10 terms = Sum of first 60 terms – Sum of first 50 terms

Since the sum of first n terms of an AP, S_n = [2 a + (n – 1) d]

∴ Sum of first 60 terms, S₆₀ = [2 (8) + (60 – 1) (2)]

∴ S₆₀ = 30 (16 + 59 x 2) = 30 x 134

∴ S₆₀ = 4020

∴ Sum of first 50 terms, S₅₀ = [2 (8) + (50 – 1) (2)]

∴ S₆₀ = 25 (16 + 49 x 2) = 25 x 114

∴ S₆₀ = 2850

Step 3: The sum of last 10 terms = S₆₀ – S₅₀

= 4020 – 2850 = 1170

Therefore, the sum of last 10 terms of AP is 1170.

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