The aim of this article is to demonstrate the dummy variables for estimation seasonal effects in a time series, to use them as inputs in a regression model for obtaining quality predictions. Model parameters were estimated using the least square method. After fitting, special tests to determine, if the model is satisfactory, were employed. The application data. Show Solution. Since this is just the tangent line there really isn't a whole lot to finding the linear approximation . f ′ ( x) = 1 3 x − 2 3 = 1 3 3 √ x 2. This online calculator uses several regression models for approximation of an unknown function given by a set of data points.