a) $\mathop {\lim }\limits_{x \to 0}\frac{1-\cos 6x}{x^{2}}$
b) $\mathop {\lim }\limits_{x \to 0}\frac{1-\cos 3x}{1-\cos 5x}$
c) $\mathop {\lim }\limits_{x \to \frac{\Pi }{2}}\frac{\cos x}{x-\frac{\Pi }{2}}$
d) $\mathop {\lim }\limits_{x \to 0}\frac{\sqrt{2x+1}-\sqrt[3]{x^{2}+1}}{\sin x}$