(b) Show that f(x) is not differentiable at x = 0.

II、計算問答題(每大題 10 分，共 20 分。) 1. Mary has T = 40 observations on variables y, $x_1$, and $x_2$. She estimates the following equations by least squares: Residual sum of squares (SSR)=140, $R^2 = 0.8$ where the numbers in parentheses are standard errors. What is the true value of SST in equation (1)? What is the value of $s^2$ (estimated error variance.)?