Exercise 3
If you wanted to use the method of least squares to fit an equation of the form
$x^2 + y^2 + z^2 = ax + by + cz + d$
to some data $(x_1, y_1, z_1), \dots, (x_D, y_D, z_D)$, explicitly write down the normal equations for
a, b, c, d. Don't solve the system, only write it down.
