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Question #40778

Difficulty: 8/10 | 🟢 Free

The Question

Let u^,v^,w^\hat{u}, \hat{v}, \hat{w} be three mutually orthogonal unit vectors in R3\mathbb{R}^3. Let S1,S2,S_1, S_2, and S3S_3 be sets of points defined as follows: S1={P∈R3:∣OP⃗∣=2 and ∣OP⃗⋅u^∣=2}S_1 = \{ P \in \mathbb{R}^3 : |\vec{OP}| = 2 \text{ and } |\vec{OP} \cdot \hat{u}| = 2 \} S2={Q∈R3:∣OQ⃗∣=3 and ∣OQ⃗⋅v^∣=3}S_2 = \{ Q \in \mathbb{R}^3 : |\vec{OQ}| = 3 \text{ and } |\vec{OQ} \cdot \hat{v}| = 3 \} S3={R∈R3:∣OR⃗∣=6 and ∣OR⃗⋅w^∣=6}S_3 = \{ R \in \mathbb{R}^3 : |\vec{OR}| = 6 \text{ and } |\vec{OR} \cdot \hat{w}| = 6 \} where OO is the origin. Let KK be the convex polyhedron whose vertices are all the points in S1∪S2∪S3S_1 \cup S_2 \cup S_3. Find the sum of the squares of the areas of all the faces of KK.

Options

  • A.

    1008

  • B.

    126

  • C.

    504

  • D.

    252

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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

Condition of Collinearity|Dot Product Projection Sign|Direction Cosines constraint

Why This Question Is Tricky

Common Mistakes

The question tests the condition of collinearity using the dot product projection and the critical trap of sign ambiguity.

Students often solve ∣OP⃗⋅u^∣=∣OP⃗∣|\vec{OP} \cdot \hat{u}| = |\vec{OP}| by incorrectly assuming OP⃗=∣OP⃗∣u^\vec{OP} = |\vec{OP}|\hat{u}, forgetting the anti-parallel case OP⃗=−∣OP⃗∣u^\vec{OP} = -|\vec{OP}|\hat{u}.

This error causes the solution space to collapse from 6 points (forming a 3D octahedron) to just 3 points, which are incorrectly identified as merely forming a single coplanar triangle.

Consequently, they calculate the area squared of only one face (126) instead of the full polyhedron (1008).

Quick Facts About This Question

Question Information

SubjectMathematics
ChapterStraight Lines - Class 11
ConceptCondition of Collinearity
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree