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Question

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

A
10d
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B
a+20d
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C
a+10d
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D
20d
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Solution

The correct option is C a+10d
a(a+d)+(a+2d)+...... upto 21 terms
last term =a+20d
Sum of 1st and 2nd and 3rd & 4th =d
So, till 20 terms =d
Sum =10d
Sum upto 21 terms =10d+a+20d
=a+10d (Answer) (C)

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