The total number is irrational terms in the binomial expansion of

Question:

The total number is irrational terms in the binomial expansion of $\left(7^{1 / 5}-3^{1 / 10}\right)^{60}$ is:

  1. (1) 55

  2. (2) 49

  3. (3) 48

  4. (4) 54


Correct Option: , 4

Solution:

Let the general term of the expansion

$T_{r+1}={ }^{60} C_{r}\left(7^{\frac{1}{5}}\right)^{60-r}\left(-3^{\frac{1}{10}}\right)^{r}$

$={ }^{60} C_{r} \cdot(7)^{12-\frac{r}{5}}(-1)^{r} \cdot(3)^{\frac{r}{10}}$

Then, for getting rational terms, $r$ should be multiple of L.C.M. of $(5,10)$

Then, $r$ can be $0,10,20,30,40,50,60$.

Since, total number of terms $=61$

Hence, total irrational terms $=61-7=54$

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