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Question

The sum of an A.P series is k(a+ccb), whose last two terms are b and c respectively. If a be the first term of the series, then the value of k is

A
2c+ba2
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B
2c+b+a2
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C
2cba2
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D
2cb+a2
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Solution

The correct option is C 2cba2
T1=a, Tn1=b, Tn=cd=TnTn1=cbc=a+(n1)d=a+(n1)(cb)n=2cbacb

Sn=k(a+ccb)n2(T1+Tn)=k(a+ccb)(2cba2(cb))×(a+c)=k(a+ccb)k=2cba2

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