Tìm giới hạn
$a) \mathop {\lim }\limits_{x \to 0}(\frac{2}{\sin 2x}-\cot x ) $
$b) \mathop {\lim }\limits_{x \to 1} \frac{\sqrt{x+3}-2x }{\tan (x-1)} $
$c)\mathop {\lim }\limits_{x \to 0}[\tan 2x \tan (\frac{\pi}{4}-x )] $
$d)\mathop {\lim }\limits_{x \to 0}[\frac{98}{83}(\frac{1-\cos 3x \cos 5x \cos 7x}{\sin^2 7x} )]$