If the coefficient of 7th and 13th term in the expansion of (1+x)n are equal, then n=
A
10
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B
15
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C
18
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D
20
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Solution
The correct option is C 18 (x+a)n=xn+Cn1xn−1a+...+Cnrxn−rar+...anSincethe(r+1)thtermisCnrxn−rar.Hencethecoefficentof7thtermisCn6and13thtermisCn12Cn6=Cn12⟹n=18Henceoption(C)