For what values of [math]a[/math] and [math]b[/math] is [math] \lim_{x\to 0} \left (\frac{\tan {2x} }{x^3} + \frac{a}{x^2} + \frac {\sin {bx} }{x} \right) = 0 [/math] ? How do I calculate [math]\lim\limits_{x\to0}\frac{\sqrt{1+\tan x}-\sqrt{1+x}}{\sin^2 x}[/math]?