Ehi, you need to define function f again in refine() to make it work. In other words:
function points = refine(x1, x2, tol)
f = @(x) (exp(-x/2).*sin(x.^2+8));
...
Reason begin, f is only in scope of linearinterp(). It is not in scope for refine() and therefore unknown. Alternatively, you can pass f as an input argument to refine():
points = refine(0, 7, 0.6, f)
...
function points = refine(x1, x2, tol, f)
In that case you do not need to define f again.
