JEE Challenger
More from Complex Numbers and Quadratic Equations

Evaluate Truth Value of Complex Number Inequalities

Let θ1,θ2,…,θ10\theta_1, \theta_2, \dots, \theta_{10} be positive-valued angles (in radians) satisfying θ1+θ2+⋯+θ10=2π\theta_1 + \theta_2 + \dots + \theta_{10} = 2\pi. Define complex numbers z1=eiθ1z_1 = e^{i\theta_1} and zk=zk−1eiθkz_k = z_{k-1}e^{i\theta_k} for k=2,3,…,10k = 2, 3, \dots, 10, where i=−1i = \sqrt{-1}. Examine the statements PP and QQ below:

P:∣z2−z1∣+∣z3−z2∣+⋯+∣z10−z9∣+∣z1−z10∣≤2πP : |z_2 - z_1| + |z_3 - z_2| + \dots + |z_{10} - z_9| + |z_1 - z_{10}| \le 2\pi

Q:∣z22−z12∣+∣z32−z22∣+⋯+∣z102−z92∣+∣z12−z102∣≤4πQ : |z_2^2 - z_1^2| + |z_3^2 - z_2^2| + \dots + |z_{10}^2 - z_9^2| + |z_1^2 - z_{10}^2| \le 4\pi

Then,

Options

A

PP is TRUE and QQ is FALSE

B

QQ is TRUE and PP is FALSE

C

both PP and QQ are TRUE

Correct
D

both PP and QQ are FALSE

Step-by-Step Solution

We analyze the statements PP and QQ using the geometric properties of complex numbers on the unit circle.

For statement PP, each term ∣zk−zk−1∣|z_k - z_{k-1}| represents the chord length on the unit circle corresponding to the angle θk\theta_k. Using the inequality sin⁡x<x\sin x < x for x>0x > 0, we have: ∣zk−zk−1∣=2sin⁡(θk2)<θk|z_k - z_{k-1}| = 2\sin\left(\frac{\theta_k}{2}\right) < \theta_k Summing these chord lengths over all 10 segments gives: ∑k=210∣zk−zk−1∣+∣z1−z10∣<∑k=110θk=2π\sum_{k=2}^{10} |z_k - z_{k-1}| + |z_1 - z_{10}| < \sum_{k=1}^{10} \theta_k = 2\pi Thus, statement PP is TRUE.

For statement QQ, using the identity ∣zk2−zk−12∣=2sin⁡θk|z_k^2 - z_{k-1}^2| = 2\sin\theta_k along with sin⁡θk≤θk\sin\theta_k \le \theta_k for θk>0\theta_k > 0, the total sum yields: ∑k=210∣zk2−zk−12∣+∣z12−z102∣=∑k=1102sin⁡θk≤2∑k=110θk=4π\sum_{k=2}^{10} |z_k^2 - z_{k-1}^2| + |z_1^2 - z_{10}^2| = \sum_{k=1}^{10} 2\sin\theta_k \le 2\sum_{k=1}^{10} \theta_k = 4\pi Thus, statement QQ is TRUE.

Hence, both PP and QQ are TRUE, which corresponds to Option (C).