Let $R_1$ and $R_2$ respectively be the maximum ranges up and down an inclined plane and $R$ be the maximum range on the horizontal plane. Then $R_1,R,R_2$ are in:

Maximum range on horizontal plane $R=\frac{u^2}{g}$.
Maximum range up inclined plane (angle $\alpha$): $R_1=\frac{u^2}{g(1+\sin \alpha )}$.
Maximum range down inclined plane: $R_2=\frac{u^2}{g(1-\sin \alpha )}$.
Check the relationship:
$$\frac{1}{R_1}+\frac{1}{R_2}=\frac{g(1+\sin \alpha )}{u^2}+\frac{g(1-\sin \alpha )}{u^2}=\frac{2g}{u^2}=\frac{2}{R}$$
This is the condition for $R_1,R,R_2$ to be in Harmonic Progression (H.P.).