A projectile of mass m is fired from ground with speed u at an...

${v}_{x} = ucos \theta = vcos \dfrac{\theta}{2}$ , where $v$ is the speed at this instant

${v}_{y} = vsin\left(\dfrac{\theta}{2}\right) = ucos\theta\dfrac{\sin \left(\dfrac{\theta}{2}\right)}{\cos \left(\dfrac{\theta}{2}\right)} = ucos\theta tan\left(\dfrac{\theta}{2}\right)$

$P = \overset{\to}{F} . \overset{\to}{v} = - mgucos\theta tan\left(\dfrac{\theta}{2}\right)$