Question

If sum of the series $a-(a+d)+(a+2d)-(a+3d)+$......up to $21$ terms is

• A. $-10d$
• B. $a+20d$
• C. $a+10d$
• D. $20d$

Answer 1

The correct option is C $a+10d$

$a-(a+d)+(a+2d)+......$ upto $21$ terms

last term $=a+20d$

Sum of $1^{st}$ and $2^{nd}$ and $3^{rd}$ & $4^{th}$ $=-d$

So, till $20$ terms $=-d$

Sum $=-10d$

Sum upto $21$ terms $=-10d+a+20d$

$=a+10d$ (Answer) (C)