(II) Let two linear waves be represented by $D_{1}=f_{1}(x, t)$ and $D_{2}=f_{2}(x, t) .$ If both these waves satisfy the wave equation (Eq. $15-16$ ), show that any combination $D=C_{1} D_{1}+C_{2} D_{2}$ does as well, where $C_{1}$ and $C_{2}$ are constants.