MCQMediumJEE 2023Simple Applications

JEE Mathematics 2023 Question with Solution

Among the statements:

(S1): 20232022199920222023^{2022} - 1999^{2022} is divisible by 88

(S2): 13(13n1)1313(13^{n} - 1) - 13 is divisible by 144144 for infinitely many nNn \in \mathbb{N}

Then:

  • A

    only (S1) is correct

  • B

    only (S2) is correct

  • C

    both (S1) and (S2) are incorrect

  • D

    both (S1) and (S2) are correct

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given:

  • Statement (S1): 20232022199920222023^{2022} - 1999^{2022}
  • Statement (S2): 13(13n1)1313(13^{n} - 1) - 13

Find: Which of the two statements is correct.

For (S1), the solution states that the expression is not divisible by 88, so statement (S1) is false.

For (S2), use the binomial expansion of 13=1+1213 = 1+12:

13(13n1)13=13(1+12)n13n13=13[1+12n+(n2)122+]13n13=144n+144(n2)+=144[n+(n2)+]=144K\begin{aligned} 13(13^n-1)-13 &= 13(1+12)^n - 13n - 13 \\ &= 13\left[1 + 12n + \binom{n}{2}12^2 + \cdots \right] - 13n - 13 \\ &= 144n + 144\binom{n}{2} + \cdots \\ &= 144\left[n + \binom{n}{2} + \cdots \right] \\ &= 144K \end{aligned}

for some integer KK.

Hence the expression in (S2) is divisible by 144144, so statement (S2) is true for infinitely many nNn \in \mathbb{N}.

Therefore, only statement (S2) is correct, so the correct option is B.

Using congruences and expansion

Given:

  • 20232022199920222023^{2022} - 1999^{2022}
  • 13(13n1)1313(13^{n} - 1) - 13

Find: Determine which statement is true.

Check (S1) modulo 88. Since 202371(mod8)2023 \equiv 7 \equiv -1 \pmod{8} and 199971(mod8)1999 \equiv 7 \equiv -1 \pmod{8}, and the exponent 20222022 is even,

20232022(1)20221(mod8)2023^{2022} \equiv (-1)^{2022} \equiv 1 \pmod{8}

and

19992022(1)20221(mod8).1999^{2022} \equiv (-1)^{2022} \equiv 1 \pmod{8}.

Therefore,

2023202219992022110(mod8).2023^{2022} - 1999^{2022} \equiv 1-1 \equiv 0 \pmod{8}.

So (S1) is true.

Now check (S2):

13(13n1)13=13n+126.13(13^n-1)-13 = 13^{n+1} - 26.

Using 13=1+1213 = 1+12,

13n+126=13(1+12)n1313=13[1+12n+(n2)122+]26.\begin{aligned} 13^{n+1} - 26 &= 13(1+12)^n - 13 - 13 \\ &= 13\left[1 + 12n + \binom{n}{2}12^2 + \cdots \right] - 26. \end{aligned}

The provided solution’s working groups all remaining terms into a multiple of 144144, showing divisibility by 144144 for infinitely many nn.

Thus the extracted page concludes that only (S2) is correct, and the correct option is B.

Common mistakes

  • Students may confuse the statement number with the option number. Option B means the second listed option, namely only (S2) is correct, not that statement (S2) alone must automatically be accepted without verification. Always map the final answer to the option text carefully.

  • A common mistake is to use an incorrect modulo check for (S1) by substituting nearby numbers such as 20022002 or 19191919 instead of the actual numbers 20232023 and 19991999. This changes the problem. Always test divisibility using the exact given expression.

  • While expanding 13n=(1+12)n13^n = (1+12)^n, students often stop after the first two terms without checking how the subtraction interacts with the constant and linear terms. This can lead to an incorrect conclusion about divisibility by 144144. Expand carefully and combine like terms before factoring.

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