Tích phân $I=\displaystyle\displaystyle\int\limits_{\tfrac{\pi}{4}}^{\tfrac{\pi}{3}}\dfrac{\mathrm{d}x}{\sin^2x}$ bằng

	$\cot\dfrac{\pi}{3}-\cot\dfrac{\pi}{4}$
	$\cot\dfrac{\pi}{3}+\cot\dfrac{\pi}{4}$
	$-\cot\dfrac{\pi}{3}+\cot\dfrac{\pi}{4}$
	$-\cot\dfrac{\pi}{3}-\cot\dfrac{\pi}{4}$