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TS EAMCET · Maths · Trigonometric Ratios & Identities

If \(A=\sin ^2 \theta+\cos ^4 \theta\), then for all values of \(\theta, A\) lies in the interval

  1. A \([1,2]\)
  2. B \(\left[\frac{3}{4}, 1\right]\)
  3. C \(\left[\frac{1}{2}, \frac{3}{4}\right]\)
  4. D \(\left[\frac{3}{4}, \frac{19}{16}\right]\)
Verified Solution

Answer & Solution

Correct Answer

(B) \(\left[\frac{3}{4}, 1\right]\)

Step-by-step Solution

Detailed explanation

We have, \[ \begin{aligned} A & =\sin ^2 \theta+\cos ^4 \theta \\ & =1-\cos ^2 \theta+\cos ^4 \theta \\ & =1-\cos ^2 \theta\left(1-\cos ^2 \theta\right) \\ & =1-\cos ^2 \theta \sin ^2 \theta \\ & =1-\frac{1}{4} \sin ^2 2 \theta \end{aligned} \] We know that,…