# Write an exponential function from a table

The student does not understand the basic form of an exponential function. Examples of Student Work at this Level The student attempts to write a linear function or an exponential expression. Questions Eliciting Thinking What is an exponential function? What does the equation of an exponential function look like? Bankers and people interested in rates of growth prefer this form. Assume that t is measured in years. This is an exponential growth function expressed in rate form. In the sketch below we have shown that the slope of the curve is 0.

This is an exponential decay function expressed in time constant form. In the sketch we have shown this for two different intervals.

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Substituting this into the original exponential function gives This form clearly shows that y decays by a factor of 9 every 2 seconds. Instead we can equate the given original form for y with the desired form. This gives We must solve this equation for T. Now divide both sides by t and solve for T.

Substituting this into the desired form gives This form clearly shows that y decays by a factor of every 4. Using the exponential form of the change of base formula and some simple algebra it is possible to rewrite this function in the following equivalent forms: Discuss the merits of each of the forms.

All of the forms have a base greater than 1 so the negative exponent indicates exponential decay as opposed to growth.

All of the forms have y with an initial value of Form a shows that y decays by a factor of 4 each time t increases by 1 second.

Form b shows that y decays by a factor of 2 each time t increases by 0. Form c shows that y decays at an instantaneous rate of In other words y decays with a time constant of 0.

Note that these statements hold anywhere along the exponential curve.

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Compounded continuously is just another way of saying grows exponentially. Let y denote the amount of money in the account at any time t. The units of y will also be dollars and t must be given in years so that the units of r and t cancel.

This value on the t axis is the time constant. Our function has a time constant of 20 so put that on the t axis. Algebra Coach Exercises Analyzing Exponential Growth and Decay Just as 2 points determine a straight line, so 2 points determine an exponential function. To derive its equation follow these steps: The electric current, i, flowing in a certain electric circuit decays exponentially with time, t, as shown.Section Exponential Growth and Decay Functions Identifying Graphs of Exponential Functions Work with a partner.

Match each exponential function with its graph. Use a table of values to sketch the graph of the function, if necessary. Jul 08,  · To write an exponential function given a rate and an initial value, start by determining the initial value and the rate of interest.

For example if a bank account was opened with \$ at an annual interest rate of 3%, the initial value is and the rate is%(1).

Identifying Exponential Functions from a Table ­ A function is said to be an exponential function if equal steps in the independent variable produce equal ratios for. Jul 08,  · To write an exponential function given a rate and an initial value, start by determining the initial value and the rate of interest. For example if a bank account was opened with \$ at an annual interest rate of 3%, the initial value is and the rate is%(1). Then, the function must be something like y = a + b Log[3-x]. Now, since you have two parameters (a and b), just use two of the data points of your graph; this .

Construct a table for taxi rides ranging from 0 to 8 miles. Construct a graph using the data in the table. Write a function that models the fare as a function of distance. 4. Dimension 2 CROSSCUTTING CONCEPTS.

Some important themes pervade science, mathematics, and technology and appear over and over again, whether we are looking at an ancient civilization, the human body, or a comet.

## Finding the equation of an exponential function from a table

Write exponential functions of the basic form f(x)=a⋅rˣ, either when given a table with two input-output pairs, or when given the graph of the function. Make a table for the function using the given domain. Then graph the function using You can write an equation for an exponential function (x) =f ab x by finding or calculating the values of a and b.

The value of a is the value of the function when =x 0. The value of b is the common ratio of successive function values, b = ƒ (x + 1) _____.

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