Let g be a twice-differentiable function with g(1) = 7 and g(7) = 1 and let f(x) = g(g(x)). a) Justify why there must be a value c, where 1 < c < 7, such that g' (c) = −1. b) Justify that f′ (1) = f′(7) and that there exists a value n such that on (1,7) where f''(n) = 0

Respuesta :

Item a):

The Mean Value Theorem states that if a function f is continuous on the closed interval [a,b] and differentiable on the open interval (a,b), then there exists a point c in the interval (a,b) such that f'(c) is equal to the function's average rate of change over [a,b].

[tex]f^{\prime}(c)=\frac{f(b)-f(a)}{b-a}[/tex]

Applying this theorem in our problem, we have:

[tex]g^{\prime}(c)=\frac{g(7)-g(1)}{7-1}=\frac{1-7}{6}=-\frac{6}{6}=-1[/tex]

item b):

f(x) is given by the composition of g(x) with itself, therefore, we can calculate the derivative of f(x) by using the chain rule:

[tex]f^{\prime}(x)=g^{\prime}(g(x))\cdot g^{\prime}(x)[/tex]

Then, evaluating this expression at x = 1 and x = 7, we have:

[tex]\begin{gathered} f^{\prime}(1)=g^{\prime}(g(1))\cdot g^{\prime}(1)=g^{\prime}(7)\cdot g^{\prime}(1) \\ f^{\prime}(7)=g^{\prime}(g(7))\cdot g^{\prime}(7)=g^{\prime}(1)\cdot g^{\prime}(7) \end{gathered}[/tex]

Thus

[tex]f^{\prime}(1)=f^{\prime}(7)[/tex]

Using the Leibniz rule, we can find the second derivative of f(x):

[tex]f^{\prime}^{\prime}(x)=g^{\prime}^{\prime}(g(x))(g^{\prime}(x))^2+g^{\prime}(g(x))g^{\prime}^{\prime}(x)[/tex]

When those two terms are antisymmetric, the second derivative is equal to zero.