نکته: $\underset{x\to 0}{\mathop{\lim }}\,\frac{\operatorname{Sin}x}{x}=1$ نکته: $\operatorname{Cos}2x=2{{\operatorname{Cos}}^{2}}x-1$ $\underset{x\to 0}{\mathop{\lim }}\,f(x)=\underset{x\to 0}{\mathop{\lim }}\,\frac{2{{\operatorname{Cos}}^{2}}x-1{{\operatorname{Cos}}^{2}}x}{{{x}^{2}}}=\underset{x\to 0}{\mathop{\lim }}\,\frac{{{\operatorname{Cos}}^{2}}x-1}{{{x}^{2}}}=\underset{x\to 0}{\mathop{\lim }}\,\frac{-{{\operatorname{Sin}}^{2}}x}{{{x}^{2}}}=\underset{x\to 0}{\mathop{\lim }}\,(\frac{-\operatorname{Sin}x}{x}\times \frac{\operatorname{Sin}x}{x})=-1$ صفحۀ ۱۴۴ حسابان ۱