Degree of canonical bundle

123 Views Asked by At

I'm reading theorem IV.5.3 in Hartshorne book and he says : let $f: X \longrightarrow X^{\prime}$ be a canonical morphism of hypereliptic curve. Suppose $\mu=deg(f)\geq2$ and $d=deg(X^{\prime})$ than $d\mu = 2g-2$, because $degK_X=2g-2$. Why does this product equal degree of canonical class?