High School Math : Understanding Derivatives of Exponents

Study concepts, example questions & explanations for High School Math

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Example Questions

Example Question #1 : Specific Derivatives

Find the derivative \(\displaystyle y^{'}\)for \(\displaystyle y = x^{2}2^x\)

Possible Answers:

\(\displaystyle x2^{x+1}+\ln(2)x^{2}2^{x}\)

\(\displaystyle 2x\ln(2)2^{x}\)

\(\displaystyle 2x2^{x}-x^22^{x-1}\)

\(\displaystyle 2x2^{x}+x^{3}2^{x-1}\)

Correct answer:

\(\displaystyle x2^{x+1}+\ln(2)x^{2}2^{x}\)

Explanation:

The derivative must be computed using the product rule.  Because the derivative of \(\displaystyle x^2\) brings a \(\displaystyle 2\) down as a coefficient, it can be combined with \(\displaystyle 2^x\) to give \(\displaystyle 2^{x+1}\)

Example Question #2142 : High School Math

Give the instantaneous rate of change of the function \(\displaystyle f (x) = \left ( \frac{1}{3} \right ) ^{x}\) at \(\displaystyle x = 3\).

Possible Answers:

\(\displaystyle -27\ln 3\)

\(\displaystyle \frac{ \ln 3}{27}\)

\(\displaystyle -\frac{ \ln 3}{27}\)

\(\displaystyle \frac{ 1}{27}\)

\(\displaystyle -\frac{ 1}{27}\)

Correct answer:

\(\displaystyle -\frac{ \ln 3}{27}\)

Explanation:

The instantaneous rate of change of \(\displaystyle f\) at \(\displaystyle x = x _{0}\) is \(\displaystyle f ' (x _{0})\), so we will find \(\displaystyle f ' (x)\) and evaluate it at \(\displaystyle x = 3\).

\(\displaystyle \frac{\mathrm{d}}{\mathrm{d} x} \left ( a ^{x } \right ) = \ln a \cdot a ^{x }\) for any positive \(\displaystyle a\), so 

\(\displaystyle f'(x) =\frac{\mathrm{d}}{\mathrm{d} x}\left [ \left ( \frac{1}{3} \right ) ^{x }\right ] = \ln \frac{1}{3} \cdot \left ( \frac{1}{3} \right) ^{x }= -\frac{ \ln 3}{3^{x}}\)

\(\displaystyle f'(3) =-\frac{ \ln 3}{3^{3}} = -\frac{ \ln 3}{27}\)

Example Question #2 : Specific Derivatives

\(\displaystyle f(x) = 5 ^{x + 1}\)

What is \(\displaystyle f'(x)\) ?

Possible Answers:

\(\displaystyle f'(x) = \ln 5 \cdot 5 ^{x}\)

\(\displaystyle f'(x) = \frac{5 ^{x+1}}{\ln 5}\)

\(\displaystyle f'(x) = \frac{5 ^{x}}{\ln 5}\)

\(\displaystyle f'(x) = \ln 5 \cdot 5 ^{x+1}\)

\(\displaystyle f'(x) = 5 ^{x}\)

Correct answer:

\(\displaystyle f'(x) = \ln 5 \cdot 5 ^{x+1}\)

Explanation:

\(\displaystyle f(x) = 5 ^{x + 1} = 5 ^{1} \cdot 5 ^{x } = 5 \cdot 5 ^{x }\)

Therefore, 

\(\displaystyle f'(x) = 5 \cdot \frac{\mathrm{d} }{\mathrm{d} x} \left ( 5 ^{x } \right )\)

\(\displaystyle \frac{\mathrm{d}}{\mathrm{d} x} \left ( a ^{x } \right ) = \ln a \cdot a ^{x }\) for any real \(\displaystyle a\), so \(\displaystyle \frac{\mathrm{d} }{\mathrm{d} x} \left ( 5 ^{x } \right ) = \ln 5 \cdot 5 ^{x}\), and

\(\displaystyle f'(x) = 5 \ln 5 \cdot 5 ^{x} = \ln 5 \cdot 5 \cdot 5 ^{x} = \ln 5 \cdot 5 ^{x+1}\)

Example Question #62 : Derivatives

\(\displaystyle f(x) = 6 ^{x - 2}\)

What is \(\displaystyle f'(4)\) ?

Possible Answers:

\(\displaystyle \frac{\ln 6}{6}\)

\(\displaystyle 6 \ln 6\)

\(\displaystyle \ln 36\)

\(\displaystyle \ln 6\)

\(\displaystyle 36 \ln 6\)

Correct answer:

\(\displaystyle 36 \ln 6\)

Explanation:

\(\displaystyle f(x) = 6 ^{x -2} = 6 ^{-2} \cdot 6 ^{x } = \frac{1}{36} \cdot 6 ^{x }\)

Therefore, 

\(\displaystyle f'(x) = \frac{1}{36} \cdot \frac{\mathrm{d} }{\mathrm{d} x} \left ( 6 ^{x } \right )\)

\(\displaystyle \frac{\mathrm{d}}{\mathrm{d} x} \left ( a ^{x } \right ) = \ln a \cdot a ^{x }\) for any positive \(\displaystyle a\), so \(\displaystyle \frac{\mathrm{d} }{\mathrm{d} x} \left ( 6 ^{x } \right ) = \ln 6 \cdot 6 ^{x}\), and

\(\displaystyle f'(x) = \frac{1}{36} \ln 6 \cdot 6 ^{x} = \ln 6 \cdot \frac{1}{36} \cdot 6 ^{x}\)

\(\displaystyle f'(x) = \ln 6 \cdot 6 ^{x-2}\)

 

\(\displaystyle f'(4) = \ln 6 \cdot 6 ^{4-2} = \ln 6 \cdot 6 ^{2} = 36 \ln 6\)

 

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