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1²-2²+3²-4²+5²-6²+ $\cdot \cdot \cdot$ +19²-20²

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270

–150

+240

–210

answer is D.

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Detailed Solution

=(1+2)(1–2) + (3+4)(3-4).......+ (19+20)(19–20)
= – (3 + 7 + .... + 39)

= -10/2(3 + 39)

= –210

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