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### Using Polynomial Regression Analysis

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The trendlines you've seen so far have all been unidirectional. That's fine if the curve formed by the dependent variable values is also unidirectional, but that's often not the case in a business environment. Sales fluctuate, profits rise and fall, and costs move up and down, thanks to varying factors such as inflation, interest rates, exchange rates, and commodity prices. For these more complex curves, the trendlines covered so far might not give either a good fit or good forecasts.

If that's the case, you might need to turn to a polynomial trendline, which is a curve constructed out of an equation that uses multiple powers of x. For example, a second-order polynomial regression equation takes the following general form: