ACT Math : How to graph an exponential function

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #1 : How To Graph An Exponential Function

Give the \(\displaystyle x\)-intercept of the graph of the function

\(\displaystyle f(x) = 5 \cdot 4^{x- 3}- 3\)

Round to the nearest tenth, if applicable.

Possible Answers:

\(\displaystyle ( -3.08, 0)\)

\(\displaystyle (3.65,0)\)

The graph has no \(\displaystyle x\)-interceptx

\(\displaystyle (2.63,0)\)

\(\displaystyle ( -2.92, 0)\)

Correct answer:

\(\displaystyle (2.63,0)\)

Explanation:

The \(\displaystyle x\)-intercept is \(\displaystyle (a,0)\), where \(\displaystyle f(a)= 0\):

\(\displaystyle f(x) = 5 \cdot 4^{x- 3}- 3\)

\(\displaystyle 5 \cdot 4^{a- 3}- 3 = 0\)

\(\displaystyle 5 \cdot 4^{a- 3}- 3 + 3= 0 + 3\)

\(\displaystyle 5 \cdot 4^{a- 3}= 3\)

\(\displaystyle 5 \cdot 4^{a- 3} \div 5= 3 \div 5\)

\(\displaystyle 4^{a- 3} = 0.6\)

\(\displaystyle \ln \left (4^{a- 3} \right )=\ln 0.6\)

\(\displaystyle (a- 3)\ln 4 =\ln 0.6\)

\(\displaystyle a- 3=\frac{\ln 0.6}{\ln 4 } \approx \frac{-0.5108}{1.3863}\approx -0.3685\)

\(\displaystyle a- 3+3 \approx -0.3685 + 3\)

\(\displaystyle a \approx 2.63\)

The \(\displaystyle x\)-intercept is \(\displaystyle (2.63,0)\).

Example Question #71 : Graphing

Give the \(\displaystyle y\)-intercept of the graph of the function

\(\displaystyle f(x) = 5 \cdot 4^{x- 3}- 3\)

Round to the nearest hundredth, if applicable.

Possible Answers:

\(\displaystyle (0, -2.92)\)

\(\displaystyle (0,3.65)\)

\(\displaystyle (0, -3.08)\)

The graph has no \(\displaystyle y\)-intercept

\(\displaystyle ( 0, 2.63)\)

Correct answer:

\(\displaystyle (0, -2.92)\)

Explanation:

The \(\displaystyle y\)-intercept is \(\displaystyle (0, f(0))\):

\(\displaystyle f(x) = 5 \cdot 4^{x- 3}- 3\)

\(\displaystyle f(0) = 5 \cdot 4^{0- 3}- 3\)

\(\displaystyle f(0) = 5 \cdot 4^{ - 3}- 3\)

\(\displaystyle f(0) = 5 \cdot \frac{1}{64}- 3\)

\(\displaystyle f(0) = \frac{5}{64}- 3\)

\(\displaystyle f(0) \approx 0.08 - 3 \approx -2.92\)

\(\displaystyle (0, -2.92)\) is the \(\displaystyle y\)-intercept.

Example Question #2 : How To Graph An Exponential Function

Give the vertical asymptote of the graph of the function 

\(\displaystyle g(x) = 16 \cdot 4^{x}- 3\)

Possible Answers:

\(\displaystyle x = -3\)

\(\displaystyle y = -2\)

\(\displaystyle y = 2\)

\(\displaystyle x = 3\)

The graph of \(\displaystyle g\) has no vertical asymptote.

Correct answer:

The graph of \(\displaystyle g\) has no vertical asymptote.

Explanation:

Since 4 can be raised to the power of any real number, the domain of \(\displaystyle f\) is the set of all real numbers. Therefore, there is no vertical asymptote of the graph of \(\displaystyle f\).

Example Question #131 : Advanced Geometry

Give the horizontal asymptote of the graph of the function 

\(\displaystyle g(x) = 9 \cdot 3^{x}- 3\)

Possible Answers:

\(\displaystyle y = -2\)

\(\displaystyle y = 3\)

The graph has no horizontal asymptote.

\(\displaystyle y = 2\)

\(\displaystyle y = -3\)

Correct answer:

\(\displaystyle y = -3\)

Explanation:

We can rewrite this as follows:

\(\displaystyle g(x) = 9 \cdot 3^{x}- 3\)

\(\displaystyle g(x) = 3^{2} \cdot 3^{x}- 3\)

\(\displaystyle g(x) = 3^{x+2}- 3\)

This is a translation of the graph of \(\displaystyle f(x) = 3^{x}\), which has \(\displaystyle y = 0\) as its horizontal asymptote, to the right two units and down three units. Because of the latter translation, the horizontal asymptote is \(\displaystyle y = -3\).

Example Question #72 : Graphing

If the functions 

\(\displaystyle f(x) = e^{x}+ 9\)

\(\displaystyle g(x)= 4e^{x}- 7\)

were graphed on the same coordinate axes, what would be the \(\displaystyle y\)-coordinate of their point of intersection?

Round to the nearest tenth, if applicable.

Possible Answers:

The graphs of \(\displaystyle f\) and \(\displaystyle g\) would not intersect.

\(\displaystyle 5.3\)

\(\displaystyle 2.7\)

\(\displaystyle 1.7\)

\(\displaystyle 14.3\)

Correct answer:

\(\displaystyle 14.3\)

Explanation:

We can rewrite the statements using \(\displaystyle y\) for both \(\displaystyle f(x)\) and \(\displaystyle g(x)\) as follows:

\(\displaystyle y = e^{x}+ 9\)

\(\displaystyle y = 4e^{x}- 7\)

To solve this, we can multiply the first equation by \(\displaystyle -4\), then add:

\(\displaystyle -4y = -4e^{x}-36\)

      \(\displaystyle \underline{y = 4e^{x} \; \; - 7}\)

\(\displaystyle -3y =\)            \(\displaystyle -43\)

\(\displaystyle -3y \div (-3) = -43 \div (-3 )\)

\(\displaystyle y \approx 14.3\)

Example Question #1 : How To Graph An Exponential Function

If the functions 

\(\displaystyle f(x) = e^{x}+ 9\)

\(\displaystyle g(x) = 4e^{x}- 7\)

were graphed on the same coordinate axes, what would be the \(\displaystyle x\)-coordinate of their point of intersection?

Round to the nearest tenth, if applicable.

Possible Answers:

\(\displaystyle 1.7\)

\(\displaystyle 5.3\)

\(\displaystyle 14.3\)

The graphs of \(\displaystyle f\) and \(\displaystyle g\) would not intersect.

\(\displaystyle 2.7\)

Correct answer:

\(\displaystyle 1.7\)

Explanation:

We can rewrite the statements using \(\displaystyle y\) for both \(\displaystyle f(x)\) and \(\displaystyle g(x)\) as follows:

\(\displaystyle y = e^{x}+ 9\)

\(\displaystyle y = 4e^{x}- 7\)

To solve this, we can set the expressions equal, as follows:

\(\displaystyle 4e^{x}- 7 = e^{x}+ 9\)

\(\displaystyle 4e^{x}- 7 - e^{x} + 7 = e^{x}+ 9 - e^{x} + 7\)

\(\displaystyle 3e^{x} = 16\)

\(\displaystyle 3e^{x}\div 3 = 16 \div 3\)

\(\displaystyle e^{x} = \approc 5.3333\)

\(\displaystyle x \approx \ln 5.333 \approx 1.7\)

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