Question : Find the least value of $16 \operatorname{cosec}^2 \theta+25 \sin ^2 \theta$
Option 1: 40
Option 2: 42
Option 3: 35
Option 4: 38
Correct Answer: 40
Solution :
To find the least value of the expression \(16 \operatorname{cosec}^2\theta + 25 \sin^2 \theta\), we can apply the AM-GM inequality:
The arithmetic mean is always greater or equal to the geometric mean.
⇒ $\frac{16 \operatorname{cosec}^2 \theta + 25 \sin^2 \theta}{2} \geq \sqrt{(16\operatorname{cosec}^2 \theta)(25 \sin^2 \theta)}$
Solving this inequality, we get:
⇒ $16 \operatorname{cosec}^2 \theta + 25 \sin^2 \theta \geq 2 \sqrt{16 \times 25} \geq 40$
Therefore, the least value of \(16 \operatorname{cosec}^2\theta + 25 \sin^2 \theta\) is \(40\).
Hence, the correct answer is 40.
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