Start Learning Free

Before You Begin

Question #41347

Difficulty: 8/10 | 🟢 Free

The Question

Let S={z∈C:Im(z2)=32 and ∣z+4+4i∣≤42}S = \{z \in \mathbb{C} : \text{Im}(z^2) = 32 \text{ and } |z + 4 + 4i| \le 4\sqrt{2}\}. A particle PP representing the complex number z=x+iyz = x+iy moves along the locus SS such that its imaginary part increases at a constant rate of 44 units/sec. What is the rate of change of ∣z∣2|z|^2 at the instant when ∣Re(z)∣=2|\text{Re}(z)| = 2?

Options

  • A.

    -60

  • B.

    60

  • C.

    -68

  • D.

    68

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Monster

Ultra-high difficulty questions designed to test your limit, stamina, and deep conceptual understanding.

Concepts Behind This Question

Rate of Change Dependency|Rectangular Hyperbola|Geometry of Complex Inequalities

Why This Question Is Tricky

Common Mistakes

This question tests the intersection of complex loci geometry with differential calculus.

The core trap lies in the geometric constraint imposed by the complex inequality ∣z+4+4i∣≤42|z + 4 + 4i| \le 4\sqrt{2}.

Students often mechanically solve Im(z2)=32  ⟹  xy=16\text{Im}(z^2) = 32 \implies xy = 16 and, upon seeing ∣Re(z)∣=2|\text{Re}(z)| = 2, default to the positive first-quadrant branch (x=2,y=8x=2, y=8).

If they use the positive branch, the chain rule yields dxdt=−1\frac{dx}{dt} = -1, and the rate of change of ∣z∣2|z|^2 becomes 2(2)(−1)+2(8)(4)=602(2)(-1) + 2(8)(4) = 60.

By enforcing the circle's interior condition, the problem strictly isolates the third-quadrant branch (x=−2,y=−8x=-2, y=-8).

This flips the signs of the coordinates but keeps the rates the same, resulting in a drastically different final value (−60-60).

Quick Facts About This Question

Question Information

SubjectMathematics
ChapterApplication of Derivatives - Class 12
ConceptRate of Change Dependency
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree