Question:
The sum of first five multiples of 3 is:
(a) 45
(b) 65
(c) 75
(d) 90
Solution:
We have to find the sum of first five multiples of 3
First five multiples of 3 are 3, 6, 9, 12 and 15
It forms an Arithmetic Progression (A.P) with
First termĀ
Common difference, $d=a_{2}-a$
$=6-3$
$=3$
We know that the sum of the $n$ terms of an arithmetic progression $\mathrm{S}_{n}=\frac{n}{2}(2 a+(n-1) d)$
Here $n=5$
Therefore
$\mathrm{S}_{5}=\frac{5}{2}(2 \times 3+(5-1) 3)$
$=\frac{5}{2} \times 18$
$=45$
Hence option (a) is correct.
Click here to get exam-ready with eSaral
For making your preparation journey smoother of JEE, NEET and Class 8 to 10, grab our app now.