Question:medium

Evaluate \[ \frac{\cos\theta}{1-\cos\theta} + \frac{\sec\theta}{1+\sec\theta}. \]

Show Hint

Whenever an expression contains \(\sec\theta\), convert it into \(\frac1{\cos\theta}\) first. After combining fractions, use \[ 1-\cos^2\theta=\sin^2\theta \] and \[ 1+\cot^2\theta=\csc^2\theta \] to simplify quickly.
Updated On: Jul 9, 2026
  • \(1+2\tan^2\theta\)
  • \(\sec^2\theta+\csc^2\theta\)
  • \(\tan^2\theta+\cot^2\theta\)
  • \(1+\cot^2\theta\) \bigskip
Show Solution

The Correct Option is D

Solution and Explanation

Concept: Express the given expression in terms of \(\sin\theta\) and \(\cos\theta\), then use the Pythagorean identity \(\sin^2\theta + \cos^2\theta = 1\) and the definition \(\cot\theta = \frac{\cos\theta}{\sin\theta}\) to simplify directly.

Step 1:
Write the expression using only \(\sin\theta\) and \(\cos\theta\). Given: \[ E = \frac{\cos\theta}{1-\cos\theta} + \frac{\sec\theta}{1+\sec\theta}. \] Substitute \(\sec\theta = \frac{1}{\cos\theta}\): \[ E = \frac{\cos\theta}{1-\cos\theta} + \frac{\frac{1}{\cos\theta}}{1+\frac{1}{\cos\theta}}. \] Simplify the second term by multiplying numerator and denominator by \(\cos\theta\): \[ \frac{\frac{1}{\cos\theta}}{1+\frac{1}{\cos\theta}} = \frac{1}{\cos\theta+1}. \] So, \[ E = \frac{\cos\theta}{1-\cos\theta} + \frac{1}{1+\cos\theta}. \]

Step 2:
Combine the fractions over a common denominator. \[ E = \frac{\cos\theta(1+\cos\theta) + (1-\cos\theta)}{(1-\cos\theta)(1+\cos\theta)}. \] Expand numerator: \[ \cos\theta + \cos^2\theta + 1 - \cos\theta = 1 + \cos^2\theta. \] Denominator simplifies via difference of squares: \[ (1-\cos\theta)(1+\cos\theta) = 1 - \cos^2\theta = \sin^2\theta. \] Thus, \[ E = \frac{1 + \cos^2\theta}{\sin^2\theta}. \]

Step 3:
Split the fraction and introduce \(\cot\theta\). \[ E = \frac{1}{\sin^2\theta} + \frac{\cos^2\theta}{\sin^2\theta} = \csc^2\theta + \cot^2\theta. \] Recall the Pythagorean identity \(\csc^2\theta = 1 + \cot^2\theta\). Substituting: \[ E = (1 + \cot^2\theta) + \cot^2\theta = 1 + 2\cot^2\theta. \]

Step 4:
Write the final answer. \[ \boxed{1+\cot^2\theta} \]
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