Show that $\sin x$ lies between $x-x^3/6$ and $x \;$ $\forall x \in R$ 
Show that $\sin x$ lies between $x-x^3/6$ and $x \;$ $\forall x \in R$

I am getting:
$$\sin(x) = x - \frac{x^3}{3!} + R_4(x)$$
where $R_4(x) = \frac{\cos(c)x^5}{5!}$ for some $c$ between $0$ and $x$ 
I want to prove $R_4(x)\geq 0$ to arrive at the result $x-x^3/3 \leq \sin(x)$.
for:$$0 \leq x \leq \pi/2 \Rightarrow 0<c<\pi/2 \Rightarrow R_4(x)\geq 0$$ but for: $$-\pi/2 \leq x < 0 \Rightarrow -\pi/2 < c < 0  \Rightarrow R_4(x) < 0$$
How can I proceed with this ? There are many cases that I need to check

This is an easy consequence of Taylor's theorem. We have via Taylor's theorem with Langrange's form of remainder $$\sin x=x-\frac{x^{3}}{3!}\cos\theta x$$ where $0<\theta<1$. Since $\cos\theta x\leq 1$ it follows that $$\sin x\geq x-\frac{x^{3}}{6},\,\forall x\geq 0$$ The inequality $\sin x\leq x$ for $x\geq 0$ is a fundamental property of $\sin x$ which is an immediate consequence of any suitable definition of $\sin x$. Thus for example the geometrical definition based on arc length of a circle implies $$x=\int_{0}^{\sin x} \frac{dt} {\sqrt{1-t^{2}}} $$ for $x\in(-\pi/2,\pi/2)$ and gives the inequality directly as the integrand is greater than or equal to $1$.

More generally if $x\geq 0$ then Taylor's theorem can be used to show that $$x-\frac{x^{3}}{3!}+\dots+(-1)^{n-1}\frac{x^{2n-1}}{(2n-1)!}\leq \sin x\leq x-\frac{x^{3}}{3!}+\dots+(-1)^{n}\frac{x^{2n+1}}{(2n+1)!}$$ whenever $n$ is an even positive integer. 
