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In tanα = √a, where a is a rational number which is not a perfect square, then which of the following is a rational number?
  • a)
    sin2α
  • b)
    tan2α
  • c)
    cos2α
  • d)
    none
Correct answer is option 'C'. Can you explain this answer?
Most Upvoted Answer
In tanα= √a, where a is a rational number which is not a p...
Understanding the Problem
Given the equation tan(α) = √a, where 'a' is a rational number that is not a perfect square, we need to explore which of the trigonometric values (sin²α, tan²α, cos²α) is a rational number.
Key Relationships
- Trigonometric Identities:
- tan²α = sin²α / cos²α
- sin²α + cos²α = 1
Analysis of tan²α
- Calculate tan²α:
- tan²α = a (since tan(α) = √a)
- Since 'a' is rational, tan²α is rational.
Analysis of sin²α
- Using the identity:
- sin²α = tan²α * cos²α
- We know tan²α is rational, but cos²α must be evaluated to determine sin²α's rationality.
Analysis of cos²α
- Using the sine and cosine relationship:
- cos²α = 1 / (1 + tan²α)
- cos²α = 1 / (1 + a)
- Since 'a' is rational, 1 + a is also rational.
- Therefore, cos²α = 1 / (1 + a) is a rational number.
Conclusion
- Final Evaluation:
- sin²α depends on the value of cos²α, which is rational.
- tan²α is rational.
- cos²α is rational.
Thus, the correct answer is option 'C', as cos²α is the only confirmed rational number when tan(α) = √a and 'a' is a non-perfect square rational number.
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