1) \(f\left(x\right)=2x-5\)

\(f'\left(x\right)=2\)

\(\Rightarrow f'\left(4\right)=2\)

2) \(y=x^2-3\sqrt[]{x}+\dfrac{1}{x}\)

\(\Rightarrow y'=2x-\dfrac{3}{2\sqrt[]{x}}-\dfrac{1}{x^2}\)

3) \(f\left(x\right)=\dfrac{x+9}{x+3}+4\sqrt[]{x}\)

\(\Rightarrow f'\left(x\right)=\dfrac{1.\left(x+3\right)-1.\left(x+9\right)}{\left(x-3\right)^2}+\dfrac{4}{2\sqrt[]{x}}\)

\(\Rightarrow f'\left(x\right)=\dfrac{x+3-x-9}{\left(x-3\right)^2}+\dfrac{2}{\sqrt[]{x}}\)

\(\Rightarrow f'\left(x\right)=\dfrac{12}{\left(x-3\right)^2}+\dfrac{2}{\sqrt[]{x}}\)

\(\Rightarrow f'\left(x\right)=2\left[\dfrac{6}{\left(x-3\right)^2}+\dfrac{1}{\sqrt[]{x}}\right]\)

\(\Rightarrow f'\left(1\right)=2\left[\dfrac{6}{\left(1-3\right)^2}+\dfrac{1}{\sqrt[]{1}}\right]=2\left(\dfrac{3}{2}+1\right)=2.\dfrac{5}{2}=5\)

\(\begin{array}{l}\Delta x = x - {x_0} = x - 1\\\Delta y = f({x_0} + \Delta x) - f({x_0}) = f(x) - f(1)\\\mathop {\lim }\limits_{x \to 1} \frac{{\Delta y}}{{\Delta x}} = \mathop {\lim }\limits_{x \to 1} \frac{{f(x) - f(1)}}{{x - 1}} = \mathop {\lim }\limits_{x \to 1} \frac{{3{x^3} - 1 - (3 - 1)}}{{x - 1}} = \mathop {\lim }\limits_{x \to 1} \frac{{3{x^3} - 3}}{{x - 1}}\\ = \mathop {\lim }\limits_{x \to 1} \frac{{3(x - 1)({x^2} + x + 1)}}{{x - 1}} = \mathop {\lim }\limits_{x \to 1} (3({x^2} + x + 1)) = 9\end{array}\)

Vậy \(f'(1) = 9\)

\(f'\left( x \right) =  - \frac{1}{{{{\sin }^2}x}} \Rightarrow f'\left( { - \frac{\pi }{3}} \right) =  - \frac{1}{{{{\sin }^2}\left( { - \frac{\pi }{3}} \right)}} =  - \frac{4}{3}\)

a) \(f'\left( 1 \right) = \mathop {\lim }\limits_{x \to 1} \frac{{f\left( x \right) - f\left( 1 \right)}}{{x - 1}} = \mathop {\lim }\limits_{x \to 1} \frac{{{x^2} - x}}{{x - 1}} = \mathop {\lim }\limits_{x \to 1} \frac{{x\left( {x - 1} \right)}}{{x - 1}} = \mathop {\lim }\limits_{x \to 1} x = 1\)

Vậy \(f'\left( 1 \right) = 1\)

b) \(f'\left( { - 1} \right) = \mathop {\lim }\limits_{x \to  - 1} \frac{{f\left( x \right) - f\left( { - 1} \right)}}{{x + 1}} = \mathop {\lim }\limits_{x \to  - 1} \frac{{ - {x^3} - 1}}{{x + 1}} = \mathop {\lim }\limits_{x \to  - 1} \frac{{ - \left( {x + 1} \right)\left( {{x^2} - x + 1} \right)}}{{x + 1}} = \mathop {\lim }\limits_{x \to  - 1} \left( {{x^2} - x + 1} \right) = 3\)

Vậy \(f'\left( { - 1} \right) = 3\)

Xét \(\Delta x\) là số gia của biến số tại điểm x

Ta có:

\(\begin{array}{l}\Delta y = f\left( {x + \Delta x} \right) - f\left( x \right) = {\left( {x + \Delta x} \right)^3} - {x^3} = \left( {x + \Delta x - x} \right)\left[ {x{{\left( {x + \Delta x} \right)}^2} + x.\left( {x + \Delta x} \right) + {x^2}} \right]\\ = \Delta x\left( {{x^2} + 2x.\Delta x + {{\left( {\Delta x} \right)}^2} + {x^2} + x.\Delta x + {x^2}} \right) = \Delta x.\left( {3{x^2} + {{\left( {\Delta x} \right)}^2} + 3x.\Delta x} \right)\\ \Rightarrow \frac{{\Delta y}}{{\Delta x}} = 3{x^2} + {\left( {\Delta x} \right)^2} + 3x.\Delta x\end{array}\)

Ta thấy:

\(\begin{array}{l}\mathop {\lim }\limits_{\Delta x \to 0} \frac{{\Delta y}}{{\Delta x}} = \mathop {\lim }\limits_{\Delta x \to 0} \left( {3{x^2} + {{\left( {\Delta x} \right)}^2} + 3x.\Delta x} \right) = 3{x^2}\\ \Rightarrow f'\left( x \right) = 3{x^2}\end{array}\)

\(\begin{array}{l}f'\left( x \right) = {\left( {\sqrt x } \right)'} = \frac{1}{{2\sqrt x }}\\ \Rightarrow f'\left( 9 \right) = \frac{1}{{2\sqrt 9 }} = \frac{1}{{2.3}} = \frac{1}{6}\end{array}\)

\(f'\left( x \right) = {10^x}.\ln 10 \Rightarrow f'\left( { - 1} \right) = {10^{ - 1}}.\ln 10 = \frac{{\ln 10}}{{10}}\)

\(f'\left( x \right) = \frac{1}{{{{\cos }^2}x}} \Rightarrow f'\left( { - \frac{\pi }{6}} \right) = \frac{1}{{{{\cos }^2}\left( { - \frac{\pi }{6}} \right)}} = \frac{4}{3}\)

\(f'\left(x\right)=\dfrac{1}{x\cdot ln10}\)

=>\(f'\left(\dfrac{1}{2}\right)=\dfrac{1}{\dfrac{1}{2}\cdot ln10}=\dfrac{2}{ln10}\)