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NDA 2025
Exam
NDA
Year
2025
Subject
Mathematics
Chapter
Vector Algebra

NDA 2025 · Mathematics · Vector Algebra

Consider the following statements in respect of a vector d = ( a ×...

Consider the following statements in respect of a vector d⃗=(a⃗×b⃗)×c⃗\vec{d} = (\vec{a} \times \vec{b}) \times \vec{c}:
I. d⃗\vec{d} is coplanar with a⃗\vec{a} and b⃗\vec{b}.
II. d⃗\vec{d} is perpendicular to c⃗\vec{c}.
Which of the statements given above is/are correct?

Options

  1. AI only
  2. BII only
  3. CBoth I and II
  4. DNeither I nor II

Correct answer

C. Both I and II

Step-by-step solution

Given d⃗=(a⃗×b⃗)×c⃗\vec{d} = (\vec{a} \times \vec{b}) \times \vec{c}

Using the vector triple product expansion:

d⃗=(a⃗⋅c⃗)b⃗−(b⃗⋅c⃗)a⃗\vec{d} = (\vec{a} \cdot \vec{c})\vec{b} - (\vec{b} \cdot \vec{c})\vec{a}

This shows that d⃗\vec{d} is expressed as a linear combination of a⃗\vec{a} and b⃗\vec{b}. Thus, d⃗\vec{d} lies in the plane containing a⃗\vec{a} and b⃗\vec{b}, making it coplanar with a⃗\vec{a} and b⃗\vec{b}. Statement I is correct.

By the definition of the cross product, the vector resulting from the cross product of two vectors is perpendicular to both of them. Since d⃗\vec{d} is the cross product of (a⃗×b⃗)(\vec{a} \times \vec{b}) and c⃗\vec{c}, it must be perpendicular to both (a⃗×b⃗)(\vec{a} \times \vec{b}) and c⃗\vec{c}.

Therefore, d⃗⋅c⃗=0\vec{d} \cdot \vec{c} = 0, which means d⃗\vec{d} is perpendicular to c⃗\vec{c}. Statement II is correct.

Both statements I and II are correct.

Answer: Both I and II
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