The solutions for Question 3.3 give the effective mass as
m_{eff} = \frac{\hbar^2}{2a^2} \frac{1}{2t cos(ka) + 4t'cos(2ka)}
I believe this should be
m_{eff} = \frac{\hbar^2}{2a^2} \frac{1}{t cos(ka) + 4t'cos(2ka)}
because the factor of 2 before t cos(ka) was already put in front of the second fraction.
Furthermore, the plot in 3.3 has a wrong x-axis labels. Currently, it says ka runs from -\pi / a to \pi / a, but it should be either that k runs between these values or that ka runs from -\pi to \pi. The plot in 3.4 does correctly have k from -\pi / a to \pi / a.