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JEE Advanced 2018 Paper 2 · Mathematics · Complex Numbers · hard

Let ω=e^2π i/5. List-I (P) Σk=1^4|1-ω^k|^2 (Q) prodk=1^4(1-ω^k (R) Σk=0^4(1+ω^k^7 (S) prodk=0^4(2-ω^k List-II (1) 5 (2) 10 (3) 110 (4) 31 (5) 0

Single correct option

Let ω=e2πi/5\omega=e^{2\pi i/5}. Match the quantities in List-I with the values in List-II.

List-I

(P) ∑k=14∣1−ωk∣2\displaystyle\sum_{k=1}^{4}\left|1-\omega^{k}\right|^{2}

(Q) ∏k=14(1−ωk)\displaystyle\prod_{k=1}^{4}\left(1-\omega^{k}\right)

(R) ∑k=04(1+ωk)7\displaystyle\sum_{k=0}^{4}\left(1+\omega^{k}\right)^{7}

(S) ∏k=04(2−ωk)\displaystyle\prod_{k=0}^{4}\left(2-\omega^{k}\right)

List-II

(1) 55

(2) 1010

(3) 110110

(4) 3131

(5) 00
  1. A. P→2, Q→1, R→4, S→3
  2. B. P→1, Q→2, R→3, S→4
  3. C. P→2, Q→5, R→3, S→4
  4. D. P→2, Q→1, R→3, S→4
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Answer: (D) P→2, Q→1, R→3, S→4

(P) ∣1−ωk∣2=2−2cos⁡2πk5|1-\omega^{k}|^{2}=2-2\cos\frac{2\pi k}{5}, and ∑k=14cos⁡2πk5=−1\sum_{k=1}^{4}\cos\frac{2\pi k}{5}=-1, so P=8+2=10P=8+2=10, which is (2). (Q) The roots of x5−1x−1=1+x+x2+x3+x4\frac{x^{5}-1}{x-1}=1+x+x^{2}+x^{3}+x^{4} are ωk\omega^{k}, k=1,…,4k=1,\dots,4; evaluating at x=1x=1 gives ∏(1−ωk)=5\prod(1-\omega^{k})=5, which is (1). (R) Expanding, ∑k(1+ωk)7=∑j(7j)∑kωjk\sum_k(1+\omega^{k})^{7}=\sum_j\binom7j\sum_k\omega^{jk} and the inner sum is 55 when 5∣j5\mid j and 00 otherwise, so R=5[(70)+(75)]=5(1+21)=110R=5\left[\binom70+\binom75\right]=5(1+21)=110, which is (3). (S) Since ∏k=04(x−ωk)=x5−1\prod_{k=0}^{4}(x-\omega^{k})=x^{5}-1, putting x=2x=2 gives 3131, which is (4). Hence P→2, Q→1, R→3, S→4. Numerical evaluation with complex arithmetic gives 1010, 55, 110110, 3131, all matching. Option (5) is unused. Notice that all four parts use the identity x5−1=∏k(x−ωk)x^{5}-1=\prod_{k}(x-\omega^{k}) or the orthogonality of roots of unity. A frequent error in (R) is to include more binomial terms than the multiples of five, or to forget the factor five from the sum over kk.
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