Consider the function $f$ defined on $\mathbb { R }$ by :
$$f ( x ) = \frac { 1 } { 1 + \mathrm { e } ^ { - 3 x } }$$
We denote $\mathscr { C } _ { f }$ its representative curve in an orthogonal coordinate system of the plane. We name A the point with coordinates $\left( 0 ; \frac { 1 } { 2 } \right)$ and B the point with coordinates $\left( 1 ; \frac { 5 } { 4 } \right)$. Below we have drawn the curve $\mathscr { C } _ { f }$ and $\mathscr { T }$ the tangent line to the curve $\mathscr { C } _ { f }$ at the point with abscissa 0.
Part A: graphical readings
In this part, results will be obtained by graphical reading. No justification is required.
- Determine the reduced equation of the tangent line $\mathscr { T }$.
- Give the intervals on which the function $f$ appears to be convex or concave.
Part B : study of the function
- We admit that the function $f$ is differentiable on $\mathbb { R }$.
Determine the expression of its derivative function $f ^ { \prime }$.
2. Justify that the function $f$ is strictly increasing on $\mathbb { R }$.
3. a. Determine the limit at $+ \infty$ of the function $f$. b. Determine the limit at $- \infty$ of the function $f$.
4. Determine the exact value of the solution $\alpha$ of the equation $f ( x ) = 0,99$.
Part C : Tangent line and convexity
- Determine by calculation an equation of the tangent line $\mathscr { T }$ to the curve $\mathscr { C } _ { f }$ at the point with abscissa 0.
We admit that the function $f$ is twice differentiable on $\mathbb { R }$. We denote $f ^ { \prime \prime }$ the second derivative function of the function $f$. We admit that $f ^ { \prime \prime }$ is defined on $\mathbb { R }$ by:
$$f ^ { \prime \prime } ( x ) = \frac { 9 \mathrm { e } ^ { - 3 x } \left( \mathrm { e } ^ { - 3 x } - 1 \right) } { \left( 1 + \mathrm { e } ^ { - 3 x } \right) ^ { 3 } } .$$
\setcounter{enumi}{1} - Study the sign of the function $f ^ { \prime \prime }$ on $\mathbb { R }$.
- a. Indicate, by justifying, on which interval(s) the function $f$ is convex. b. What does point A represent for the curve $\mathscr { C } _ { f }$ ? c. Deduce the relative position of the tangent line $\mathscr { T }$ and the curve $\mathscr { C } _ { f }$. Justify the answer.