KCET · Maths · Quadratic Equation
If \(\alpha\) and \(\beta\) are acute angles such that \(\alpha - \beta\) and \(\alpha + \beta\) satisfy the equation \(\tan^2\theta - 4\tan\theta + 1 = 0\), then \(\alpha\) and \(\beta\) are respectively,
- A \(45^\circ, 30^\circ\)
- B \(30^\circ, 45^\circ\)
- C \(30^\circ, 60^\circ\)
- D \(60^\circ, 45^\circ\)
Answer & Solution
Correct Answer
(A) \(45^\circ, 30^\circ\)
Step-by-step Solution
Detailed explanation
The roots of the equation \(\tan^2\theta - 4\tan\theta + 1 = 0\) are \(\tan(\alpha - \beta)\) and \(\tan(\alpha + \beta)\).
Sum of roots: \(\tan(\alpha - \beta) + \tan(\alpha + \beta) = 4\)
Product of roots: \(\tan(\alpha - \beta) \tan(\alpha + \beta) = 1\)
Using the tangent addition formula:
\(\tan(2\alpha) = \tan((\alpha - \beta) + (\alpha + \beta)) = \dfrac{\tan(\alpha - \beta) + \tan(\alpha + \beta)}{1 - \tan(\alpha - \beta) \tan(\alpha + \beta)}\)
Substituting the sum and product of roots:
\(\tan(2\alpha) = \dfrac{4}{1 - 1} \rightarrow \infty\)
Since \(\alpha\) is an acute angle, \(2\alpha = 90^\circ \Rightarrow \alpha = 45^\circ\).
Now, substituting \(\alpha = 45^\circ\) into the sum of roots equation:
\(\tan(45^\circ - \beta) + \tan(45^\circ + \beta) = 4\)
\(\dfrac{1 - \tan\beta}{1 + \tan\beta} + \dfrac{1 + \tan\beta}{1 - \tan\beta} = 4\)
\(\dfrac{(1 - \tan\beta)^2 + (1 + \tan\beta)^2}{1 - \tan^2\beta} = 4\)
\(\dfrac{2(1 + \tan^2\beta)}{1 - \tan^2\beta} = 4\)
\(1 + \tan^2\beta = 2 - 2\tan^2\beta\)
\(3\tan^2\beta = 1 \Rightarrow \tan\beta = \dfrac{1}{\sqrt{3}}\) (since \(\beta\) is acute)
Thus, \(\beta = 30^\circ\).
Answer: \(45^\circ, 30^\circ\)
Sum of roots: \(\tan(\alpha - \beta) + \tan(\alpha + \beta) = 4\)
Product of roots: \(\tan(\alpha - \beta) \tan(\alpha + \beta) = 1\)
Using the tangent addition formula:
\(\tan(2\alpha) = \tan((\alpha - \beta) + (\alpha + \beta)) = \dfrac{\tan(\alpha - \beta) + \tan(\alpha + \beta)}{1 - \tan(\alpha - \beta) \tan(\alpha + \beta)}\)
Substituting the sum and product of roots:
\(\tan(2\alpha) = \dfrac{4}{1 - 1} \rightarrow \infty\)
Since \(\alpha\) is an acute angle, \(2\alpha = 90^\circ \Rightarrow \alpha = 45^\circ\).
Now, substituting \(\alpha = 45^\circ\) into the sum of roots equation:
\(\tan(45^\circ - \beta) + \tan(45^\circ + \beta) = 4\)
\(\dfrac{1 - \tan\beta}{1 + \tan\beta} + \dfrac{1 + \tan\beta}{1 - \tan\beta} = 4\)
\(\dfrac{(1 - \tan\beta)^2 + (1 + \tan\beta)^2}{1 - \tan^2\beta} = 4\)
\(\dfrac{2(1 + \tan^2\beta)}{1 - \tan^2\beta} = 4\)
\(1 + \tan^2\beta = 2 - 2\tan^2\beta\)
\(3\tan^2\beta = 1 \Rightarrow \tan\beta = \dfrac{1}{\sqrt{3}}\) (since \(\beta\) is acute)
Thus, \(\beta = 30^\circ\).
Answer: \(45^\circ, 30^\circ\)
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