Q52 / 90 · JEE Main 2024 (08 Apr Shift 1) · Mathematics
If the range of $f(\theta)=\frac{\sin ^4 \theta+3 \cos ^2 \theta}{\sin ^4 \theta+\cos ^2 \theta}, \theta \in \mathbb{R}$ is $[\alpha, \beta]$, then the sum of the infinite G.P., whose first term is 64 and the common ratio is $\frac{\alpha}{\beta}$, is equal to________
Correct answer: 96
Solution
$\begin{aligned} & f(\theta)=\frac{\sin ^4 \theta+3 \cos ^2 \theta}{\sin ^4 \theta+\cos ^2 \theta} \\ & f(\theta)=1+\frac{2 \cos ^2 \theta}{\sin ^4 \theta+\cos ^2 \theta} \\ & f(\theta)=\frac{2 \cos ^2 \theta}{\cos ^4 \theta-\cos ^2 \theta+1}+1 \\ & f(\theta)=\frac{2}{\cos ^2 \theta+\sec ^2 \theta-1}+1 \\ & \left.f(\theta)\right|_{\text {min. }}=1 \\ & f(\theta)_{\text {max. }}=3 \\ & S=\frac{64}{1-1 / 3}=96\end{aligned}$
Hence, the correct answer is 96.