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
θ 1 + θ 2 + ⋯ + θ 10 = 2 π , θ k > 0 \theta_1+\theta_2+\cdots+\theta_{10}=2\pi,\quad \theta_k>0 θ 1 + θ 2 + ⋯ + θ 10 = 2 π , θ k > 0
z 1 = e i θ 1 , z k = z k − 1 e i θ k ( k = 2 , … , 10 ) z_1=e^{i\theta_1},\quad z_k=z_{k-1}\,e^{i\theta_k}\ (k=2,\ldots,10) z 1 = e i θ 1 , z k = z k − 1 e i θ k ( k = 2 , … , 10 )
P : ∑ k = 1 10 ∣ z k + 1 − z k ∣ ≤ 2 π ( z 11 : = z 1 ) P:\ \sum_{k=1}^{10}|z_{k+1}-z_k|\le 2\pi\quad (z_{11}:=z_1) P : ∑ k = 1 10 ∣ z k + 1 − z k ∣ ≤ 2 π ( z 11 := z 1 )
Q : ∑ k = 1 10 ∣ z k + 1 2 − z k 2 ∣ ≤ 4 π Q:\ \sum_{k=1}^{10}|z_{k+1}^{2}-z_k^{2}|\le 4\pi Q : ∑ k = 1 10 ∣ z k + 1 2 − z k 2 ∣ ≤ 4 π
Which option (1)–(4) is correct? \text{Which option (1)--(4) is correct?} Which option (1)–(4) is correct?
∣ z 1 ∣ = ∣ e i θ 1 ∣ = 1 |z_1|=|e^{i\theta_1}|=1 ∣ z 1 ∣ = ∣ e i θ 1 ∣ = 1
∣ z k ∣ = ∣ z k − 1 ∣ ⋅ ∣ e i θ k ∣ = ∣ z k − 1 ∣ = 1 |z_k|=|z_{k-1}|\cdot|e^{i\theta_k}|=|z_{k-1}|=1 ∣ z k ∣ = ∣ z k − 1 ∣ ⋅ ∣ e i θ k ∣ = ∣ z k − 1 ∣ = 1
⇒ ∣ z 1 ∣ = ⋯ = ∣ z 10 ∣ = 1 \Rightarrow\ |z_1|=\cdots=|z_{10}|=1 ⇒ ∣ z 1 ∣ = ⋯ = ∣ z 10 ∣ = 1
arg z k = θ 1 + ⋯ + θ k , arg z 10 = 2 π \arg z_k=\theta_1+\cdots+\theta_k,\qquad \arg z_{10}=2\pi arg z k = θ 1 + ⋯ + θ k , arg z 10 = 2 π
z 10 = e i 2 π = 1 z_{10}=e^{i2\pi}=1 z 10 = e i 2 π = 1
P : ∑ k = 1 10 ∣ z k + 1 − z k ∣ ≤ 2 π P:\ \sum_{k=1}^{10}|z_{k+1}-z_k|\le 2\pi P : ∑ k = 1 10 ∣ z k + 1 − z k ∣ ≤ 2 π
perimeter of the inscribed polygon ≤ circumference 2 π \text{perimeter of the inscribed polygon} \le \text{circumference } 2\pi perimeter of the inscribed polygon ≤ circumference 2 π
∣ z k + 1 − z k ∣ = 2 sin θ k + 1 2 |z_{k+1}-z_k|=2\sin\dfrac{\theta_{k+1}}{2} ∣ z k + 1 − z k ∣ = 2 sin 2 θ k + 1
2 sin θ 2 ≤ θ ( θ ≥ 0 ) 2\sin\dfrac{\theta}{2}\le \theta\quad (\theta\ge 0) 2 sin 2 θ ≤ θ ( θ ≥ 0 )
∑ k = 1 10 ∣ z k + 1 − z k ∣ = 2 ∑ k = 1 10 sin θ k 2 ≤ ∑ k = 1 10 θ k = 2 π \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 ∑ k = 1 10 ∣ z k + 1 − z k ∣ = 2 ∑ k = 1 10 sin 2 θ k ≤ ∑ k = 1 10 θ k = 2 π
∴ P is TRUE \therefore\ P\ \text{is TRUE} ∴ P is TRUE
Q : ∑ k = 1 10 ∣ z k + 1 2 − z k 2 ∣ ≤ 4 π Q:\ \sum_{k=1}^{10}|z_{k+1}^2-z_k^2|\le 4\pi Q : ∑ k = 1 10 ∣ z k + 1 2 − z k 2 ∣ ≤ 4 π
∣ z k 2 ∣ = ∣ z k ∣ 2 = 1 , arg ( z k 2 ) = 2 arg z k |z_k^2|=|z_k|^2=1,\qquad \arg(z_k^2)=2\arg z_k ∣ z k 2 ∣ = ∣ z k ∣ 2 = 1 , arg ( z k 2 ) = 2 arg z k
z k + 1 2 − z k 2 = z k 2 ( e 2 i θ k + 1 − 1 ) z_{k+1}^2-z_k^2=z_k^2\left(e^{2i\theta_{k+1}}-1\right) z k + 1 2 − z k 2 = z k 2 ( e 2 i θ k + 1 − 1 )
∣ e 2 i θ − 1 ∣ = 2 ∣ sin θ ∣ \left|e^{2i\theta}-1\right|=2|\sin\theta| e 2 i θ − 1 = 2∣ sin θ ∣
⇒ ∣ z k + 1 2 − z k 2 ∣ = 2 ∣ sin θ k + 1 ∣ \Rightarrow\ |z_{k+1}^2-z_k^2|=2|\sin\theta_{k+1}| ⇒ ∣ z k + 1 2 − z k 2 ∣ = 2∣ sin θ k + 1 ∣
∑ k = 1 10 ∣ z k + 1 2 − z k 2 ∣ = 2 ∑ k = 1 10 ∣ sin θ k ∣ ≤ 2 ∑ k = 1 10 θ k = 4 π \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 ∑ k = 1 10 ∣ z k + 1 2 − z k 2 ∣ = 2 ∑ k = 1 10 ∣ sin θ k ∣ ≤ 2 ∑ k = 1 10 θ k = 4 π
∴ Q is TRUE \therefore\ Q\ \text{is TRUE} ∴ Q is TRUE
θ k = π 5 : ∑ ∣ z k + 1 − z k ∣ = 10 ⋅ 2 sin π 10 ≈ 6.18 < 2 π ≈ 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 θ k = 5 π : ∑ ∣ z k + 1 − z k ∣ = 10 ⋅ 2 sin 10 π ≈ 6.18 < 2 π ≈ 6.28
∑ ∣ z k + 1 2 − z k 2 ∣ = 10 ⋅ 2 sin π 5 ≈ 11.76 < 4 π ≈ 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 ∑ ∣ z k + 1 2 − z k 2 ∣ = 10 ⋅ 2 sin 5 π ≈ 11.76 < 4 π ≈ 12.57
P TRUE , Q TRUE P\ \text{TRUE},\qquad Q\ \text{TRUE} P TRUE , Q TRUE
Option (3): both P and Q are TRUE \boxed{\text{Option (3): both } P \text{ and } Q \text{ are TRUE}} Option (3): both P and Q are TRUE