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Vector Algebra question

2007 · Shift 1 · Q34
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  5. /2007 · Shift 1 · Q34

Vector Algebra question

2007 · Shift 1 · Q34

JEE AdvancedMathematicsVector AlgebraMCQ+3 / −1
Let the vector PQ→,QR→,RS→,ST→,TU→\overrightarrow {PQ} ,\overrightarrow {QR} ,\overrightarrow {RS} ,\overrightarrow {ST} ,\overrightarrow {TU}PQ​,QR​,RS,ST,TU and UP→\overrightarrow {UP}UP, represent the sides of a regular hexagon. Statement 1 : PQ→×(RS→+ST→)e0→\overrightarrow {PQ} \times \left( {\overrightarrow {RS} + \overrightarrow {ST} } \right) e \overrightarrow 0PQ​×(RS+ST)e0 Statement 2 : PQ→×RS→=0→\overrightarrow {PQ} \times \overrightarrow {RS} = \overrightarrow 0PQ​×RS=0 and PQ→×ST→e0→\overrightarrow {PQ} \times \overrightarrow {ST} e \overrightarrow 0PQ​×STe0
  1. A
    Statement 1 is True, Statement 2 is True, Statement 2 is a CORRECT explanation for Statement 1
  2. B
    Statement 1 is True, Statement 2 is True, Statement 2 is NOT a CORRECT explanation for Statement 1
  3. C
    Statement 1 is True, Statement 2 is False
  4. D
    Statement 1 is False, Statement 2 is True
View written solutionFree

Correct answer: C

Step-by-step Solution:

1. Understanding the Geometry and Vectors

Let PQRSTU be a regular hexagon. The vectors PQ¨,¨QR¨,¨RS¨,¨ST¨,¨TU¨,¨\"{PQ}\", \"{QR}\", \"{RS}\", \"{ST}\", \"{TU}\",PQ¨​,¨​QR¨​,¨​RS¨,¨​ST¨,¨​TU¨,¨​ and UP⃗\vec{UP}UP represent its sides taken in order. Let the side length of the hexagon be 'a'.

Key properties of a regular hexagon:

  • All sides are equal in length, i.e., ∣PQ⃗∣=∣QR⃗∣=...=a|\vec{PQ}| = |\vec{QR}| = ... = a∣PQ​∣=∣QR​∣=...=a.
  • The interior angle between any two adjacent sides is 120°.
  • Opposite sides are parallel and equal in magnitude. Therefore, we have:
    • ST⃗=−PQ⃗\vec{ST} = -\vec{PQ}ST=−PQ​
    • TU⃗=−QR⃗\vec{TU} = -\vec{QR}TU=−QR​
    • UP⃗=−RS⃗\vec{UP} = -\vec{RS}UP=−RS

The question contains a symbol 'e', which is likely a typo for '≠\ne=' (not equal to). We will proceed with this standard interpretation.

2. Analyzing Statement 1

Statement 1: PQ⃗×(RS⃗+ST⃗)≠0⃗\vec{PQ} \times (\vec{RS} + \vec{ST}) \ne \vec{0}PQ​×(RS+ST)=0

First, let's simplify the term in the parenthesis using the triangle law of vector addition. For triangle RST, we have RS⃗+ST⃗=RT⃗\vec{RS} + \vec{ST} = \vec{RT}RS+ST=RT. So, Statement 1 is equivalent to checking if PQ⃗×RT⃗≠0⃗\vec{PQ} \times \vec{RT} \ne \vec{0}PQ​×RT=0.

The cross product of two non-zero vectors is zero if and only if they are parallel or anti-parallel. So, we need to determine if the vector PQ⃗\vec{PQ}PQ​ is parallel to the vector RT⃗\vec{RT}RT.

  • PQ⃗\vec{PQ}PQ​ is a side of the hexagon.
  • RT⃗\vec{RT}RT is a diagonal of the hexagon connecting vertices R and T.

Geometrically, in a regular hexagon, a side vector is not parallel to any diagonal except the main diagonals (like PS, QT, RU), and RT is not a main diagonal. Let's verify this more rigorously.

Let's place vertex P at the origin (0,0) and Q on the x-axis at (a, 0). Then PQ⃗=ai^\vec{PQ} = a\hat{i}PQ​=ai^.

The vertices can be found by rotating subsequent side vectors by 60° (exterior angle).

  • QR⃗=a(cos⁡60∘i^+sin⁡60∘j^)=a(12i^+32j^)\vec{QR} = a(\cos 60^{\circ} \hat{i} + \sin 60^{\circ} \hat{j}) = a(\frac{1}{2}\hat{i} + \frac{\sqrt{3}}{2}\hat{j})QR​=a(cos60∘i^+sin60∘j^​)=a(21​i^+23​​j^​)
  • RS⃗=a(cos⁡120∘i^+sin⁡120∘j^)=a(−12i^+32j^)\vec{RS} = a(\cos 120^{\circ} \hat{i} + \sin 120^{\circ} \hat{j}) = a(-\frac{1}{2}\hat{i} + \frac{\sqrt{3}}{2}\hat{j})RS=a(cos120∘i^+sin120∘j^​)=a(−21​i^+23​​j^​)
  • ST⃗=a(cos⁡180∘i^+sin⁡180∘j^)=−ai^\vec{ST} = a(\cos 180^{\circ} \hat{i} + \sin 180^{\circ} \hat{j}) = -a\hat{i}ST=a(cos180∘i^+sin180∘j^​)=−ai^

Now, we find the vector RT⃗\vec{RT}RT. RT⃗=RS⃗+ST⃗=a(−12i^+32j^)+(−ai^)=a(−32i^+32j^)\vec{RT} = \vec{RS} + \vec{ST} = a(-\frac{1}{2}\hat{i} + \frac{\sqrt{3}}{2}\hat{j}) + (-a\hat{i}) = a(-\frac{3}{2}\hat{i} + \frac{\sqrt{3}}{2}\hat{j})RT=RS+ST=a(−21​i^+23​​j^​)+(−ai^)=a(−23​i^+23​​j^​)

The vector PQ⃗\vec{PQ}PQ​ is purely in the i^\hat{i}i^ direction, while the vector RT⃗\vec{RT}RT has both i^\hat{i}i^ and j^\hat{j}j^​ components. Thus, PQ⃗\vec{PQ}PQ​ and RT⃗\vec{RT}RT are not parallel.

Therefore, their cross product is non-zero: PQ⃗×RT⃗=(ai^)×a(−32i^+32j^)=a2(i^×(−32i^)+i^×(32j^))=a2(0⃗+32k^)=a232k^≠0⃗\vec{PQ} \times \vec{RT} = (a\hat{i}) \times a(-\frac{3}{2}\hat{i} + \frac{\sqrt{3}}{2}\hat{j}) = a^2 (\hat{i} \times (-\frac{3}{2}\hat{i}) + \hat{i} \times (\frac{\sqrt{3}}{2}\hat{j})) = a^2 (\vec{0} + \frac{\sqrt{3}}{2}\hat{k}) = \frac{a^2\sqrt{3}}{2}\hat{k} \ne \vec{0}PQ​×RT=(ai^)×a(−23​i^+23​​j^​)=a2(i^×(−23​i^)+i^×(23​​j^​))=a2(0+23​​k^)=2a23​​k^=0

Thus, Statement 1 is True.

3. Analyzing Statement 2

Statement 2: PQ⃗×RS⃗=0⃗\vec{PQ} \times \vec{RS} = \vec{0}PQ​×RS=0 and PQ⃗×ST⃗≠0⃗\vec{PQ} \times \vec{ST} \ne \vec{0}PQ​×ST=0

This statement is a conjunction of two claims. For the entire statement to be true, both claims must be true.

  • Claim A: PQ⃗×RS⃗=0⃗\vec{PQ} \times \vec{RS} = \vec{0}PQ​×RS=0 This would mean PQ⃗\vec{PQ}PQ​ and RS⃗\vec{RS}RS are parallel. However, PQ⃗\vec{PQ}PQ​ and RS⃗\vec{RS}RS are sides of the regular hexagon separated by the side QR⃗\vec{QR}QR​. The angle between them is 120°. Since the angle is not 0° or 180°, the vectors are not parallel. Therefore, their cross product is not zero. PQ⃗×RS⃗≠0⃗\vec{PQ} \times \vec{RS} \ne \vec{0}PQ​×RS=0. So, Claim A is False.

Since one part of the conjunction is false, the entire Statement 2 is false. For completeness, let's also check the second claim.

  • Claim B: PQ⃗×ST⃗≠0⃗\vec{PQ} \times \vec{ST} \ne \vec{0}PQ​×ST=0 This would mean PQ⃗\vec{PQ}PQ​ and ST⃗\vec{ST}ST are not parallel. However, in a regular hexagon, opposite sides are parallel. PQ⃗\vec{PQ}PQ​ and ST⃗\vec{ST}ST are opposite sides. Specifically, they are anti-parallel, so ST⃗=−PQ⃗\vec{ST} = -\vec{PQ}ST=−PQ​. The cross product of parallel or anti-parallel vectors is the zero vector. PQ⃗×ST⃗=PQ⃗×(−PQ⃗)=−(PQ⃗×PQ⃗)=−0⃗=0⃗\vec{PQ} \times \vec{ST} = \vec{PQ} \times (-\vec{PQ}) = -(\vec{PQ} \times \vec{PQ}) = -\vec{0} = \vec{0}PQ​×ST=PQ​×(−PQ​)=−(PQ​×PQ​)=−0=0. So, the claim that PQ⃗×ST⃗≠0⃗\vec{PQ} \times \vec{ST} \ne \vec{0}PQ​×ST=0 is False.

Since both claims are false, Statement 2 is definitively False.

4. Conclusion

  • Statement 1 is True.
  • Statement 2 is False.

This corresponds to option C.

Final Answer: The final answer is C\boxed{C}C​

Previous

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