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Question

If a1a2a3an are in AP , then 1a1a2+1a2a3+1a3a4++1an1an equals-

A
a1a2n1
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B
n1a1+an
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C
n1a1an
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D
n1a1an
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Solution

The correct option is B n1a1an
a1,a2,...........an are in AP
Let common difference= dthena2a1=da3a2=detc.
now(1a1a2)=(1d)[(da1a2)]
=(1d)[(a2a1a1a2)]
=(1d)[(1a1)(1a2)]
similarly
(1a2a3)=(1d)[(1a2)(1a3)]
and(1an1an)=(1d)[(1an1)(1an)]
sum
=(1d)[((1a1)(1a2))+((1a2)(1a3))+......+((1an1)(1an)]
=(1d)[(1a1)(1an)]=(1d)(ana1a1an)
asan=a+(n1)dana=(n1)d
orsum=(1d)((n1)da1an)
=(n1a1an)


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