CBSE Question Paper for Class 10 Maths
2018 Question 9

Sum of an arithmetic progression. An easy 2 mark question that appeared in CBSE class 10 Maths board paper in 2018. Two parts to solving the question. Part 1: Figuring out the first term and common difference of the arithmetic progression. Part 2: Applying the formula to compute the sum of an arithmetic progression.

Question 9: Find the sum of first 8 multiples of 3.

Video Explanation

Explanatory Answer

The first 8 multiples of 3 are 3, 6, 9, 12, 15, 18, 21, and 24.

Note that the difference between any two consecutive terms of the above series is the same.
It is 3. So, the first 8 multiples of 3 will be in AP with a common difference of 3.

Sum of first n terms of an AP = \\frac{n}{2}) (2a₁ + (n - 1)d), where a₁ is the first term of AP.
The first term of this AP is the first multiple of 3 = 3

∴ Sum of first 8 multiples of 3, S₈ = \\frac{8}{2})(2 × 3 + (8 - 1) × 3)
S₈ = 4 (6 + 7 × 3) = 4 (27) = 108