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Question

If the coefficient of (2r+4)th term and (r2)th terms in the expansion of (1+x)18 are equal then r=

A
9
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B
4
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C
6
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D
3
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Solution

The correct option is C 6
Given expression is (1+x)18

Now, (2r+4)th term i.e T(2r+3)+1

T(2r+3)+1=18C2r+3(x)2r+3

And, (r2)th term i.e T(r3)+1

T(r3)+1=18Cr3(x)r3

According to the question

18C2r+3=18Cr3

As we know

nCx=nCyx+y=n

So,

2r+3+r3=18

3r=18

r=6

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