: 71.66 on 3 and 28 DF,  p-value: 2.981e-13</code></pre>
<p>If we express the regression results as an equation: <code>mpg = 49.81 + (-0.12 * hp) + (-8.22 * wt) + (0.03 * hp*wt)</code>, it will be easy to see how the three-dimensional surface above reflects various partial derivatives of the regression equation.</p>
<p>For example, if we take the partial derivative of the regression equation with respect to <code>wt</code> (i.e., the marginal effect of weight), the equation is: <code>d_mpg/d_wt = (-8.22) + 