$\operatorname{cosec} 10^{\circ}-4 \sin 70^{\circ}$ এর মান কত? (What is the value of $\left.\operatorname{cosec} 10^{\circ}-4 \sin 70^{\circ} ?\right)$

$\operatorname{cosec} 10^{\circ}-4 \sin 70^{\circ}$ $\Rightarrow \frac{1}{\sin 10}-4 \sin (90-20)$ $\Rightarrow \frac{1}{\sin 10}-4 \sin 2=0 \cos ^{\circ}$ $\Rightarrow \frac{1}{\sin 10}(1-4 \cos 20 \cdot \sin 10)$ $\Rightarrow \frac{1}{\sin 10}(1-2(2 \cos 20 \sin 10))$ $\Rightarrow \frac{1}{\sin 10}(1-2(\sin 30-\sin 10))$ $\Rightarrow \frac{1}{\sin 10}\left(1-2\left(\frac{1}{2}-\sin 20\right)\right)$ $\Rightarrow \frac{1}{\sin 10} \times(2 \sin 10)=2$ Explanation: এখানে, যে সূত্র ব্যবহার করা হয়েছে। $2 \cos A \sin B=\sin (A+B)-\sin (A-B)$