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JEE Advanced 2021 · Complex Numbers and Quadratic Equations

Let θ1,θ2,…,θ10\theta_{1}, \theta_{2}, \ldots, \theta_{10} be positive valued angles (in radian) such that θ1+θ2+…+θ10=2π\theta_{1} + \theta_{2} + \ldots + \theta_{10} = 2\pi. Define the complex numbers z1=eiθ1z_{1} = e^{i\theta_{1}}, zk=zk−1eiθkz_{k} = z_{k-1}e^{i\theta_{k}} for k=2,3,…,10k = 2, 3, \ldots, 10, where i=−1i = \sqrt{-1}. Consider the statements PP and QQ given below : P∣z2−z1∣+∣z3−z2∣+…+∣z10−z9∣+∣z1−z10∣≤2πP \quad |z_{2} - z_{1}| + |z_{3} - z_{2}| + \ldots + |z_{10} - z_{9}| + |z_{1} - z_{10}| \leq 2\pi Q∣z22−z12∣+∣z32−z22∣+…+∣z102−z92∣+∣z12−z102∣≤4πQ \quad |z_{2}^{2} - z_{1}^{2}| + |z_{3}^{2} - z_{2}^{2}| + \ldots + |z_{10}^{2} - z_{9}^{2}| + |z_{1}^{2} - z_{10}^{2}| \leq 4\pi

Answer: C. both PP and QQ are TRUE

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Written solution

Let θ₁, θ₂, …, θ₁₀ be positive valued angles (in radian) such that θ₁ + θ₂ + … + θ₁₀ = 2π. Define the complex numbers z₁ = e^(iθ₁), zₖ = zₖ₋₁e^(iθₖ) for k = 2, 3, …, 10, where i = √-1. Consider the statements P and Q given below : P |z₂ - z₁| + |z₃ - z₂| + … + |z₁₀ - z₉| + |z₁ - z₁₀| ≤ 2π Q |z₂² - z₁²| + |z₃² - z₂²| + … + |z₁₀² - z₉²| + |z₁² - z₁₀²| ≤ 4π θ1+θ2+⋯+θ10=2π,θk>0\theta_1+\theta_2+\cdots+\theta_{10}=2\pi,\quad \theta_k>0 z1=eiθ1,zk=zk−1 eiθk (k=2,…,10)z_1=e^{i\theta_1},\quad z_k=z_{k-1}\,e^{i\theta_k}\ (k=2,\ldots,10) P: ∑k=110∣zk+1−zk∣≤2π(z11:=z1)P:\ \sum_{k=1}^{10}|z_{k+1}-z_k|\le 2\pi\quad (z_{11}:=z_1) Q: ∑k=110∣zk+12−zk2∣≤4πQ:\ \sum_{k=1}^{10}|z_{k+1}^{2}-z_k^{2}|\le 4\pi Which option (1)–(4) is correct?\text{Which option (1)--(4) is correct?} ∣z1∣=∣eiθ1∣=1|z_1|=|e^{i\theta_1}|=1 ∣zk∣=∣zk−1∣⋅∣eiθk∣=∣zk−1∣=1|z_k|=|z_{k-1}|\cdot|e^{i\theta_k}|=|z_{k-1}|=1 ⇒ ∣z1∣=⋯=∣z10∣=1\Rightarrow\ |z_1|=\cdots=|z_{10}|=1 arg⁡zk=θ1+⋯+θk,arg⁡z10=2π\arg z_k=\theta_1+\cdots+\theta_k,\qquad \arg z_{10}=2\pi z10=ei2π=1z_{10}=e^{i2\pi}=1 P: ∑k=110∣zk+1−zk∣≤2πP:\ \sum_{k=1}^{10}|z_{k+1}-z_k|\le 2\pi perimeter of the inscribed polygon≤circumference 2π\text{perimeter of the inscribed polygon} \le \text{circumference } 2\pi ∣zk+1−zk∣=2sin⁡θk+12|z_{k+1}-z_k|=2\sin\dfrac{\theta_{k+1}}{2} 2sin⁡θ2≤θ(θ≥0)2\sin\dfrac{\theta}{2}\le \theta\quad (\theta\ge 0) ∑k=110∣zk+1−zk∣=2∑k=110sin⁡θk2≤∑k=110θk=2π\sum_{k=1}^{10}|z_{k+1}-z_k| = 2\sum_{k=1}^{10}\sin\dfrac{\theta_k}{2} \le \sum_{k=1}^{10}\theta_k = 2\pi ∴ P is TRUE\therefore\ P\ \text{is TRUE} Q: ∑k=110∣zk+12−zk2∣≤4πQ:\ \sum_{k=1}^{10}|z_{k+1}^2-z_k^2|\le 4\pi ∣zk2∣=∣zk∣2=1,arg⁡(zk2)=2arg⁡zk|z_k^2|=|z_k|^2=1,\qquad \arg(z_k^2)=2\arg z_k zk+12−zk2=zk2(e2iθk+1−1)z_{k+1}^2-z_k^2=z_k^2\left(e^{2i\theta_{k+1}}-1\right) ∣e2iθ−1∣=2∣sin⁡θ∣\left|e^{2i\theta}-1\right|=2|\sin\theta| ⇒ ∣zk+12−zk2∣=2∣sin⁡θk+1∣\Rightarrow\ |z_{k+1}^2-z_k^2|=2|\sin\theta_{k+1}| ∑k=110∣zk+12−zk2∣=2∑k=110∣sin⁡θk∣≤2∑k=110θk=4π\sum_{k=1}^{10}|z_{k+1}^2-z_k^2| = 2\sum_{k=1}^{10}|\sin\theta_k| \le 2\sum_{k=1}^{10}\theta_k = 4\pi ∴ Q is TRUE\therefore\ Q\ \text{is TRUE} θk=π5:∑∣zk+1−zk∣=10⋅2sin⁡π10≈6.18<2π≈6.28\theta_k=\dfrac{\pi}{5}:\quad \sum|z_{k+1}-z_k|=10\cdot 2\sin\dfrac{\pi}{10}\approx 6.18<2\pi\approx 6.28 ∑∣zk+12−zk2∣=10⋅2sin⁡π5≈11.76<4π≈12.57\sum|z_{k+1}^2-z_k^2|=10\cdot 2\sin\dfrac{\pi}{5}\approx 11.76<4\pi\approx 12.57 P TRUE,Q TRUEP\ \text{TRUE},\qquad Q\ \text{TRUE} Option (3): both P and Q are TRUE\boxed{\text{Option (3): both } P \text{ and } Q \text{ are TRUE}}

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