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What is the 5th term in the expansion of \left ( 3a+b \right )^9?

Option: 1

^9C_5 .3^4.a^{4}.b^5


Option: 2

^9C_4 .3^5.a^{5}.b^4


Option: 3

^9C_4.a^{5}.b^4


Option: 4

None of these


Answers (1)

best_answer

General term in this expansion is

 T_{r+1}=^nC_r.(3a)^{n-r}(b)^r

For fifth term we put r = 4

Thus T_{5}=^9C_4 .(3a)^{9-4}(b)^4= ^9C_4 .3^5.a^{5}.b^4

 

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Ritika Jonwal

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