Determine as expressões designatórias das funções derivadas

Funções seno, co-seno e tangente: Infinito 12 A - Parte 3 Pág. 50 Ex. 23

Enunciado

  1. Determine as expressões designatórias das funções derivadas das funções:

    a) $f:x \to \operatorname{sen} (3x) + \cos x$

    b) $g:x \to {\cos ^2}(2x)$

    c) $h:\alpha  \to \frac{{1 – \cos (3\alpha )}}{\alpha }$

    d) $i:z \to \frac{{1 – \cos (2z)}}{{1 + \cos (2z)}}$

    e) $j:t \to \cos \left( {4 – 3t} \right)$

  2. Sabendo que as funções $f$ e $g$ são deriváveis e que $g(1) = 3$, $g'(1) = 2$ e $f'(3) = 5$, determine, nos pontos indicados, o valor da derivada das funções seguintes:

    a) $f\left( {g(x)} \right)$ para $x = 1$;

    b) $f\left( {g\left( {2x + 1} \right)} \right)$ para $x = 0$.

Resolução

  1. a)
    $$\begin{array}{*{20}{l}}
    {f'(x)}& = &{\left( {\operatorname{sen} (3x) + \cos x} \right)’} \\
    {}& = &{\cos (3x) \times \left( {3x} \right)’ – \operatorname{sen} x} \\
    {}& = &{3\cos (3x) – \operatorname{sen} x}
    \end{array}$$
    b)
    $$\begin{array}{*{20}{l}}
    {g'(x)}& = &{\left( {{{\cos }^2}(2x)} \right)’} \\
    {}& = &{2 \times \cos (2x) \times \left( {\cos (2x)} \right)’} \\
    {}& = &{2\cos (2x) \times \left( { – \operatorname{sen} (2x) \times \left( {2x} \right)’} \right)} \\
    {}& = &{ – 2\left( {2\cos (2x) \times \operatorname{sen} (2x)} \right)} \\
    {}& = &{ – 2\operatorname{sen} (4x)}
    \end{array}$$
    c)
    \[\begin{array}{*{20}{l}}
    {h’\left( \alpha  \right)}& = &{{{\left( {\frac{{1 – \cos (3\alpha )}}{\alpha }} \right)}^\prime }} \\
    {}& = &{\frac{{\operatorname{sen} (3\alpha ) \times {{\left( {3\alpha } \right)}^\prime } \times \alpha  – 1 \times \left( {1 – \cos (3\alpha )} \right)}}{{{\alpha ^2}}}} \\
    {}& = &{\begin{array}{*{20}{l}}
    {\frac{{3\alpha \operatorname{sen} (3\alpha ) – 1 + \cos (3\alpha )}}{{{\alpha ^2}}},}&{\alpha  \ne 0}
    \end{array}}
    \end{array}\]
    d)
    \[\begin{array}{*{20}{l}}
    {i’\left( z \right)}& = &{{{\left( {\frac{{1 – \cos (2z)}}{{1 + \cos (2z)}}} \right)}^\prime }} \\
    {}& = &{\frac{{\operatorname{sen} (2z) \times {{\left( {2z} \right)}^\prime } \times \left( {1 + \cos (2z)} \right) + \operatorname{sen} (2z) \times {{\left( {2z} \right)}^\prime } \times \left( {1 – \cos (2z)} \right)}}{{{{\left( {1 + \cos (2z)} \right)}^2}}}} \\
    {}& = &{\frac{{2\operatorname{sen} (2z) + 2\operatorname{sen} (2z)\cos (2z) + 2\operatorname{sen} (2z) – 2\operatorname{sen} (2z)\cos (2z)}}{{{{\left( {1 + \cos (2z)} \right)}^2}}}} \\
    {}& = &{\begin{array}{*{20}{l}}
    {\frac{{4\operatorname{sen} (2z)}}{{{{\left( {1 + \cos (2z)} \right)}^2}}},}&{\alpha  \ne \frac{\pi }{2} + k\pi ,k \in \mathbb{Z}}
    \end{array}}
    \end{array}\]
    e)
    $$\begin{array}{*{20}{l}}
    {j'(t)}& = &{\left( {\cos \left( {4 – 3t} \right)} \right)’} \\
    {}& = &{ – \operatorname{sen} \left( {4 – 3t} \right) \times \left( {4 – 3t} \right)’} \\
    {}& = &{3\operatorname{sen} \left( {4 – 3t} \right)}
    \end{array}$$
    ­
  2. a)
    \[\begin{array}{*{20}{l}}
    {{{\left( {f\left( {g(x)} \right)} \right)}^\prime }\left( 1 \right)}& = &{f’\left( {g\left( 1 \right)} \right) \times g’\left( 1 \right)} \\
    {}& = &{f’\left( 3 \right) \times 2} \\
    {}& = &{5 \times 2} \\
    {}& = &{10}
    \end{array}\]
    b)
    \[\begin{array}{*{20}{l}}
    {{{\left( {f\left( {g\left( {2x + 1} \right)} \right)} \right)}^\prime }\left( 0 \right)}& = &{f’\left( {g\left( {2 \times 0 + 1} \right)} \right) \times g’\left( {2 \times 0 + 1} \right) \times {{\left( {2x + 1} \right)}^\prime }} \\
    {}& = &{f’\left( {g\left( 1 \right)} \right) \times g’\left( 1 \right) \times 2} \\
    {}& = &{f’\left( 3 \right) \times 2 \times 2} \\
    {}& = &{5 \times 2 \times 2} \\
    {}& = &{20}
    \end{array}\]
    ­

Se $f’$ e $g’$ existem e se $f \circ g$ está definida, então $$\left( {f \circ g} \right)'(x) = f’\left( {g(x)} \right) \times g'(x)$$

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