= Solution
For the valence band, $\beta_v>0$, so the electron curvature is negative. A missing electron is a positive-mass <electron hole>:
$$
\boxed{m_h^*=\frac{\hbar^2}{2\beta_v a^2}=0.7205m_e.}
$$
The quadratic expansion has $E_v(\mathbf k)\simeq E_v(0)-\beta_v a^2|\mathbf k|^2$, so each diagonal electron inverse mass is $-2\beta_v a^2/\hbar^2$. Reversing the sign to describe the missing electron gives the positive hole mass above. As in the conduction calculation, convert beta from eV to joules and use $a=5.42\times10^{-10}$ m; the result applies near the band edge.
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