To calculate Ymax using energy conservation, the initial kinetic energy (Ki) is equal to the potential energy (Uf) at the maximum height, leading to the equation (1/2)mV0^2 = mgymax. Simplifying this gives Ymax = V0^2/(2g). For the velocity at half Ymax, the kinetic energy at that point is the sum of the potential energy at half the height and the remaining kinetic energy. Using the energy conservation principle, the velocity at y = 1/2 Ymax can be found by rearranging the energy equation to yield V1/2 = sqrt(V0^2 - gYmax). This approach effectively utilizes energy conservation to derive both Ymax and the velocity at half that height.