In the next page he applies this useful theorem:

For applying theorem 6.11 our function must be also bounded but in theorem 6.13 function $f\in\mathcal{R}(\alpha)$ and we have no information about boundedness.
In the next page he applies this useful theorem:

For applying theorem 6.11 our function must be also bounded but in theorem 6.13 function $f\in\mathcal{R}(\alpha)$ and we have no information about boundedness.