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Question

If $\tan\theta = 1$ ($\theta$ an acute angle) then the value of $2 \sin\theta \cos\theta - \text{cosec}^2\theta$ is:

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
$-1$

Finding the Angle

Given that $\tan\theta = 1$ and $\theta$ is an acute angle.

The angle $\theta$ for which the tangent is 1 is $45^\circ$. So, $\theta = 45^\circ$.

Calculating Trigonometric Values

For $\theta = 45^\circ$, we find the required trigonometric values:

  • $\sin\theta = \sin(45^\circ) = \frac{1}{\sqrt{2}}$
  • $\cos\theta = \cos(45^\circ) = \frac{1}{\sqrt{2}}$
  • $\text{cosec}\theta = \frac{1}{\sin\theta} = \frac{1}{1/\sqrt{2}} = \sqrt{2}$
  • $\text{cosec}^2\theta = (\sqrt{2})^2 = 2$

Evaluating the Expression

The expression is $2 \sin\theta \cos\theta - \text{cosec}^2\theta$. Substitute the calculated values:

$ 2 \sin\theta \cos\theta - \text{cosec}^2\theta = 2 \left( \frac{1}{\sqrt{2}} \right) \left( \frac{1}{\sqrt{2}} \right) - 2 $

Simplify the expression:

$ = 2 \left( \frac{1}{2} \right) - 2 $

$ = 1 - 2 $

$ = -1 $

Thus, the value of the expression is $-1$.

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Important Questions from Trigonometric Ratios and Identities

  1. If cosec θ = 13/12, then sin θ + cos θ - tan θ is equal to:

  2. What is the value of \(\frac{3 \sin 58^{\circ}}{\cos 32^{\circ}}+\frac{3 \sin 42^{\circ}}{\cos 48^{\circ}}\) ?

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  4. If α + β = 90° and α = 2β, then the value of 3 cos 2 α - 2 sin 2 β is equal to:

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