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JEE Mathematics 2025 Question with Solution

The least value of nn for which the number of integral terms in the Binomial expansion of (7+11)n\left( \sqrt{7} + \sqrt{11} \right)^n is 183183, is:

  • A

    21962196

  • B

    21722172

  • C

    21842184

  • D

    21482148

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: We need the least value of nn for which the number of integral terms in the binomial expansion is 183183.

Find: The least integer nn.

From the solution working, consider the general term of

Tk=(nk)(713)nk(11112)kT_k = \binom{n}{k}\left(7^{\frac{1}{3}}\right)^{n-k}\left(11^{\frac{1}{12}}\right)^k

For TkT_k to be an integer, both exponents must be integers.

So the conditions are

nk3Z\frac{n-k}{3} \in \mathbb{Z}

and

k12Z\frac{k}{12} \in \mathbb{Z}

Hence,

k=12pk = 12p

for some integer pp, and nkn-k must be divisible by 33.

The number of admissible values is therefore taken as

n12+1\left\lfloor \frac{n}{12} \right\rfloor + 1

Given that the number of integral terms is 183183,

n12+1=183\left\lfloor \frac{n}{12} \right\rfloor + 1 = 183

So,

n12=182\left\lfloor \frac{n}{12} \right\rfloor = 182

Detailed Counting

Now solve the inequality coming from the floor function:

182×12n<183×12182 \times 12 \le n < 183 \times 12

That is,

2184n<21962184 \le n < 2196

Therefore, the least integer value is

n=2184n = 2184

So the correct option is C.

Note: The solution works with (713+11112)n\left(7^{\frac{1}{3}} + 11^{\frac{1}{12}}\right)^n and concludes n=2184n = 2184. The final answer on the page is C, which matches option 21842184.

Common mistakes

  • A common mistake is to count all multiples of 1212 without also checking the divisibility condition on nkn-k. A term is integral only when both exponent conditions are satisfied. Always verify every irrational power becomes an integer power.

  • Another mistake is to set n12=183\frac{n}{12} = 183 instead of using the counting formula n12+1=183\left\lfloor \frac{n}{12} \right\rfloor + 1 = 183. The extra 11 appears because counting includes k=0k = 0.

  • Students may choose 21962196 by taking the upper end of the interval 2184n<21962184 \le n < 2196. This is wrong because the question asks for the least value of nn. Always select the smallest integer satisfying the condition.

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