for (c) is at a single fixed point x=-1, I think we can just compute q(-1) and L(-1) to find error, why polynomial interpolation error formula needed?
I want you to use the formula, as this is a practice and helps understanding the use of the formula.
Isn’t the error of the linear spline interpolant given by less than equal so it gives us an upper bound, how can we conclude equality without compute it directly ?