\begin{Schunk}
\begin{Sinput}
 ps40 <- testSlopes(ps20)
\end{Sinput}
\begin{Soutput}
Values of x1 OUTSIDE this interval:
        lo         hi 
-2.1851709 -0.9066175 
cause the slope of (b1 + b2*x1)x2 to be statistically significant
\end{Soutput}
\begin{Sinput}
 plot(ps40)
\end{Sinput}
\end{Schunk}
