# Graphing exponential decay

When graphing exponential decay, it is always a good idea to make a table of values and you could look for at least 5 points that you can use to make the graph. In the table below, we use 6 points.

It is also a good idea to choose both negative and positive values for x, especially numbers that are opposites. For example, notice how we chose x = -1 and x = 1.

Graph y = 20 × 0.5x

 x 20×0.5x (x, y) -2 20×0.5-2 = 20 / 0.52 = 20/0.25 = 20 / (1/4) = 20 × 4 = 80 (-2, 80) -1 20×0.5-1 = 20 / 0.51 = 20/0.5 = 20 / (1/2) = 20 × 2 = 40 (-1, 40) 0 20×0.50 = 20 × 1 = 20 (0, 20) 1 20×0.51 = 20 × 0.5 = 20 × 1/2 = 10 (1, 10) 2 20×0.52 = 20 × 0.25 = 20 × 1/4 = 5 (2, 5) 3 20×0.53 = 20 × 0.125 = 20 × 1/8 = 2.5 (3, 2.5)

Did you make the following observations about the figure above?

• Each square in vertical axis or y-axis represents 10 units.
• Each square in horizontal axis or x-axis represents 1 unit.

## Important observations when graphing exponential decay

Now, let us see what will happen if we choose the following values for x:

x = -4, -10, -20

If x = -4, y = 20 × 0.5-4 = 20 / 0.54 = 20 / (1/2)4 = 20 / (1/16) = 20×16 = 320.

If x = -10, y = 20 × 0.5-10 = 20 / 0.510 = 20 / (1/2)10 = 20 / (1/1024) = 20×1024 = 20480.

If x = -20, y = 20 × 0.5-20 = 20 / 0.520 = 20 / (1/2)20 = 20 / (1/1048576) = 20×1048576 = 20971520.

When graphing exponential decay, as the values of x get smaller and smaller, the values of y get bigger and bigger. In other words, the values of y will approach infinity.

Now, let us see what will happen if we choose the following values for x:

x = 4, 10, 20

If x = 4, y = 20 × 0.54 = 20 × 0.0625 = 1.25

If x = 10, y = 20 × 0.510 = 20 × 0.0009765625 = 0.01953

If x = 20, y = 20 × 0.520 = 20 × 0.0000009537 = 0.000019074

As x increases, y decreases exponentially or decreases rapidly. The value of y get closer and closer to 0.

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