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Question #40883

Difficulty: 8/10 | 🟢 Free

The Question

A point P(3,1,2)P(\sqrt{3}, 1, 2) in 3D space is projected onto the xyxy-plane to obtain the point P′P'. The coordinates of P′P' are represented by the complex number Z0=x+iyZ_0 = x + iy. Z0Z_0 is rotated about the origin in the complex plane by an angle θ=12arg⁡(W)\theta = \frac{1}{2} \arg(W) to obtain the complex number Z1Z_1, where W=(−1+i3)5(1+i3)2W = \frac{(-1 + i\sqrt{3})^5}{(1 + i\sqrt{3})^2} and arg⁡(W)\arg(W) denotes the principal argument of WW such that arg⁡(W)∈(−π,π]\arg(W) \in (-\pi, \pi]. A new point QQ in 3D space is formed such that its projection on the xyxy-plane corresponds to Z1Z_1 and its zz-coordinate is the same as that of PP. If OO is the origin, find the value of ∣OP⃗×OQ⃗∣2|\vec{OP} \times \vec{OQ}|^2.

Options

  • A.

    28

  • B.

    60

  • C.

    48

  • D.

    32

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

Projection vs. Coordinate|Euler's Form and Rotations|Argument and Principal Value

Why This Question Is Tricky

Common Mistakes

This question tests the student's ability to correctly handle the principal value of the argument of a complex number before applying scalar multiplications (like halving the angle) in rotation transformations.

The trap lies in calculating arg⁡(W)\arg(W) using the properties of arguments: arg⁡(z15/z22)=5arg⁡(z1)−2arg⁡(z2)=8π/3\arg(z_1^5/z_2^2) = 5\arg(z_1) - 2\arg(z_2) = 8\pi/3.

If a student fails to reduce this to the principal branch (−π,π](-\pi, \pi] before dividing by 2, they will incorrectly use θ=4π/3\theta = 4\pi/3 instead of the correct θ=π/3\theta = \pi/3.

This leads to a completely different rotated coordinate Z1=−2iZ_1 = -2i, resulting in the incorrect point Q(0,−2,2)Q(0, -2, 2) and the trap answer of 60.

Quick Facts About This Question

Question Information

SubjectMathematics
ChapterIntroduction to Three-dimensional Geometry - Class 11
ConceptProjection vs. Coordinate
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree