Part I
We consider the function $f$ defined on $\mathbb{R}$ by
$$f(x) = x - \mathrm{e}^{-2x}$$
We call $\Gamma$ the representative curve of the function $f$ in an orthonormal coordinate system $(O; \vec{\imath}, \vec{\jmath})$.
- Determine the limits of the function $f$ at $-\infty$ and at $+\infty$.
- Study the monotonicity of the function $f$ on $\mathbb{R}$ and draw up its variation table.
- Show that the equation $f(x) = 0$ has a unique solution $\alpha$ on $\mathbb{R}$, and give an approximate value to $10^{-2}$ precision.
- Deduce from the previous questions the sign of $f(x)$ according to the values of $x$.
Part II
In the orthonormal coordinate system $(O; \vec{\imath}, \vec{\jmath})$, we call $\mathscr{C}$ the representative curve of the function $g$ defined on $\mathbb{R}$ by:
$$g(x) = \mathrm{e}^{-x}$$
The curves $\mathscr{C}$ and the curve $\Gamma$ (which represents the function $f$ from Part I) are drawn on the graph provided in the appendix which is to be completed and returned with your paper. The purpose of this part is to determine the point on the curve $\mathscr{C}$ closest to the origin O of the coordinate system and to study the tangent to $\mathscr{C}$ at this point.
- For any real number $t$, we denote by $M$ the point with coordinates $(t; \mathrm{e}^{-t})$ on the curve $\mathscr{C}$.
We consider the function $h$ which, to the real number $t$, associates the distance $OM$. We therefore have: $h(t) = OM$, that is:
$$h(t) = \sqrt{t^2 + \mathrm{e}^{-2t}}$$
a. Show that, for any real number $t$,
$$h'(t) = \frac{f(t)}{\sqrt{t^2 + \mathrm{e}^{-2t}}}$$
where $f$ denotes the function studied in Part I. b. Prove that the point A with coordinates $(\alpha; \mathrm{e}^{-\alpha})$ is the point on the curve $\mathscr{C}$ for which the length $OM$ is minimal. Place this point on the graph provided in the appendix, to be returned with your paper.
- We call $T$ the tangent to the curve $\mathscr{C}$ at A. a. Express in terms of $\alpha$ the slope of the tangent $T$.
We recall that the slope of the line (OA) is equal to $\frac{\mathrm{e}^{-\alpha}}{\alpha}$. We also recall the following result which may be used without proof: In an orthonormal coordinate system of the plane, two lines $D$ and $D'$ with slopes $m$ and $m'$ respectively are perpendicular if and only if the product $mm'$ is equal to $-1$. b. Prove that the line (OA) and the tangent $T$ are perpendicular.
Draw these lines on the graph provided in the appendix, to be returned with your paper.