MCQ
If $sin^4\,\,\alpha + 4\,cos^4\,\,\beta + 2 = 4\sqrt 2\,\,sin\,\alpha \,cos\,\beta ;$ $\alpha \,,\,\beta \, \in \,[0,\pi ],$ then $cos( \alpha + \beta)$ is equal to
- A$0$
- B$-1$
- C$\sqrt 2$
- ✓$-\sqrt 2$
$\frac{\sin ^{4} \alpha+4 \cos ^{4} \beta+1+1}{4} \geq\left(\sin ^{4} \alpha \cdot 4 \cos ^{4} \beta .1 .1\right)^{\frac{1}{4}}$
$\sin ^{4} \alpha+4 \cos ^{2} \beta+$ $2 \geq 4 \sqrt{2} \sin \alpha \cos \beta$ given that $\sin ^{4} \alpha+4 \cos ^{4} \beta+2$ $=4 \sqrt{2} \sin \alpha \cos \beta$
$\Rightarrow \mathrm{AM}=\mathrm{GM} \Rightarrow \sin ^{4} \alpha=1=4 \cos ^{4} \beta$
$\sin \alpha=\pm 1, \cos \beta=\pm \frac{1}{\sqrt{2}},$ As $\alpha, \beta \in[0, \pi]$
$\Rightarrow \sin \alpha=1, \cos \beta=\pm \frac{1}{\sqrt{2}}$
$\Rightarrow \sin \beta=\frac{1}{\sqrt{2}}$ as $\beta \in[0, \pi]$
$\cos (\alpha+\beta)-\cos (\alpha-\beta)=-2 \sin \alpha \sin \beta$
$=-\sqrt{2}$
Generate a complete, print-ready paper with questions like this in minutes — across 16+ boards, with answer keys.