= Solution
A wave packet in an <energy band> has <group velocity> $v=\hbar^{-1}dE/dk$. The semiclassical force law $\hbar\dot k=F$ then gives $\dot v=\hbar^{-2}(d^2E/dk^2)F$. The curvature therefore defines the one-dimensional <effective mass> by $1/m^*=E''/\hbar^2$. Here $E''(k)=Aa^2\cos(ka)$, so at the band minimum $k=0$,
$$
\boxed{m^*=\frac{\hbar^2}{Aa^2}>0.}
$$
It measures acceleration within the band, rather than a change in the electron's rest mass. At a band maximum the electron curvature mass can be negative; a missing electron is then described by a positively charged <hole> with a positive curvature mass.
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