Suppose we have a multiple linear regression with interaction: \[\hat{y} = \beta_0 + \beta_1 x_1 + \beta_2 x_2 + \beta_{1\times 2} x_1 x_2 \] Notice that the slope on \(x_1\) is not just \(\beta_1\), it's \(\beta_1 + \beta_{1\times 2} x_2\): \[\hat{y} ...