1.

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+2ndC. a+ndD. 2nd

Answer» 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`


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