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Forecasting Using a Graph
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Often we use some data from the past to make predictions about things that haven’t happened. This works by graphing the data we have, then extending the line into areas where we don’t have data. We usually are given an x value and asked for the y value. The following video will show an example of this kind of forecasting.

Forecast the Value of a Linear Function Using a Graph

Using the line already given, we extend the line so it passes the x values (horizontal) we’re interested in. Draw a vertical line from the x axis to the line to find the point. Trace that point in a horizontal line to the y-axis to find the y value of that point.

### Practice Problems

Use this graph to answers questions 1 and 2:
1. Using the line shown in the graph above, estimate the value of y when $$x=-4$$. (
Solution
Solution: Approximately $$7.8$$
Details:

We can’t see the graph of the line where $$x=-4$$ so we extend the line to estimate it:

Next, we approximate where $$x=−4$$ would fall on the graph. It appears that the y-value is approximately $$7.8$$ when $$x=−4$$.
)
2. Using the line shown in the graph above, estimate the value of y when $$x = 6$$. (
Solution
Solution:
Approximately $$-19.2$$
)

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