Choose the correct answer from the given four options:
The sum of first five multiples of 3 is
(A) 45
(B) 55
(C) 65
(D) 75

AcademicMathematicsNCERTClass 10

Given:

First five multiples of 3.

To do:

We have to find the sum of first five multiples of 3.

Solution:

First five multiples of 3 forms an AP, where

$a_1 = a= 3, a_2=6$

Common difference $d =a_2-a_1$

$=6-3$

$=3$

We know that,

Sum of the $n$ terms of an AP is $S_n=\frac{n}{2}[2a+(n-1)d]$

Therefore,

$S_{5}= \frac{5}{2}[2(3)+(5-1)(3)]$

$= \frac{5}{2}(6+4(3))$

$= \frac{5}{2}(18)$

$=5\times 9$

$=45$

The sum of the first 5 multiples of 3 is $45$. 

raja
Updated on 10-Oct-2022 13:27:27

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