Class CBSE Class 12 Mathematics Three Dimensional Geometry Q #665
KNOWLEDGE BASED
UNDERSTAND
1 Marks 2024 AISSCE(Board Exam) MCQ SINGLE
If \(\alpha\), \(\beta\) and \(\gamma\) are the angles which a line makes with positive directions of x, y and z axes respectively, then which of the following is not true?
(A) \(cos^{2}\alpha+cos^{2}\beta+cos^{2}\gamma=1\)
(B) \(sin^{2}\alpha+sin^{2}\beta+sin^{2}\gamma=2\)
(C) \(cos~2\alpha+cos~2\beta+cos~2\gamma=-1\)
(D) \(cos~\alpha+cos~\beta+cos~\gamma=1\)
Correct Answer: D

AI Tutor Explanation

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Step-by-Step Solution

We know that if \(\alpha\), \(\beta\), and \(\gamma\) are the angles a line makes with the x, y, and z axes respectively, then:

\[cos^2\alpha + cos^2\beta + cos^2\gamma = 1\]

This means option (A) is true.

Now, let's analyze option (B):

\[sin^2\alpha + sin^2\beta + sin^2\gamma = (1 - cos^2\alpha) + (1 - cos^2\beta) + (1 - cos^2\gamma)\] \[= 3 - (cos^2\alpha + cos^2\beta + cos^2\gamma) = 3 - 1 = 2\]

So, option (B) is also true.

Next, let's analyze option (C):

\[cos~2\alpha + cos~2\beta + cos~2\gamma = (2cos^2\alpha - 1) + (2cos^2\beta - 1) + (2cos^2\gamma - 1)\] \[= 2(cos^2\alpha + cos^2\beta + cos^2\gamma) - 3 = 2(1) - 3 = -1\]

So, option (C) is also true.

Finally, let's consider option (D):

There is no direct relationship that states \(cos~\alpha + cos~\beta + cos~\gamma = 1\). This statement is generally false.

Correct Answer: \(cos~\alpha+cos~\beta+cos~\gamma=1\)

AI Suggestion: Option D

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Pedagogical Audit
Bloom's Analysis: This is an UNDERSTAND question because the student needs to recall the relationship between direction cosines and then apply trigonometric identities to verify the given options.
Knowledge Dimension: CONCEPTUAL
Justification: The question tests the understanding of the concept of direction cosines and their relationship, rather than just recalling a formula.
Syllabus Audit: In the context of CBSE Class 12, this is classified as KNOWLEDGE. The question directly assesses knowledge of direction cosines and their properties, which is a core concept in the syllabus.