Question

The sum of the series `a-(a+d)+(a+2d)-(a+3d)+` up to `(2n+1)` terms is `-n d` b. `a+2n d` c. `a+n d` d. `2n d`
A. `-nd`
B. a+2nd
C. a+nd
D. 2nd

Answer 1

Correct Answer - C
The given series is an AGP with common ratio-1.
S=a-(a+b)+(a+2d)-(a+3d)+ . . . .+(a+2nd)
`rArr" "-S=-a+(a+d)-(a+2d)+ . . . .+(a+(2n-1)d)-(a+2nd)`
`:." "2S=a+{-d+d-d+d . . .` upto 2n terms}+(a+2nd)
`rArr" "2S=2a+2ndrArrS=a+nd`