Question

Find S1+S2+S3.....S10 given that Sn= 1+2+3+4.....n

• A. 260
• B. 240
• C. 340
• D. 220

Answer 1

The correct option is D

220

S1= 1

S2= 1+2 =3

S3=1+2+3= 6

This is of the form $\frac{n(n+1)}{2}$

We need to find $\sum \frac{n(n+1)}{2}$ where n=10

Expanding we get

$\frac{1}{2}[\sum n^2+\sum n]$

$\frac{1}{2}[\frac{n(n+1)(2n+1)}{6}+\frac{n(n+1)}{2}]$

Substitute n=10 to get the answer as $\frac{440}{2}$=220