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Question

If $\tan\theta = \frac{7}{8}$, then evaluate $\frac{(1 + \sin\theta)(1 - \sin\theta)}{(1 + \cos\theta)(1 - \cos\theta)(\cot\theta)}$:

The correct answer is
$\frac{8}{7}$

Given:

tanθ = 7/8

⇒ Assume in a right triangle:

height = 7, base = 8, hypotenuse = √113

sinθ = 7/√113, cosθ = 8/√113, cotθ = 8/7

Now the expression:

[(1 + sinθ)(1 − sinθ)] / [(1 + cosθ)(1 − cosθ)(cotθ)]

= (1 − sin²θ) / [(1 − cos²θ)(cotθ)]

= cos²θ / [sin²θ · cotθ]

= cos²θ / [sin²θ · (cosθ/sinθ)]

= cos²θ / (sinθ cosθ)

= cosθ / sinθ

= cotθ

= 8/7

Thus the answer = 8/7

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Important Questions from Trigonometric Identities

  1. What is \(\rm \frac{1+tan^2\theta}{1+cot^2\theta}-\left(\frac{1-tan\theta}{1-cot\theta}\right)^2\) equal to?

  2. If 3sin θ + 5cos θ = 5, then the value of 5sin θ - 3cos θ is equal to: 

  3. If angle C of a triangle ABC is a right angle where a, b and c are the sides opposite to the angles A, B and C respectively then what is tan A + tan B equal to?

  4. If \(\sin \left( {A - B} \right) = \frac{1}{2}\)  and  \(\cos \left( {A + B} \right) = \frac{1}{2}\) , where A > B > 0° and A + B is an acute angle, then the value of A is:

  5. In the equation

    \(\rm\cos^{-1} \dfrac{1-a^2}{1 + a^2} - \cos^{-1} \dfrac{1-b^2}{1 + b^2} = 2 tan^{-1} x\) value of x is

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