合速度:V=√(Vx^2+Vy^2)
=√[(V0/2-gt)^2+(gt)^2]
=√[1/4V0^2-V0gt+2g^2t^2)
与水平方向的夹角:α=arctan(Vy/Vx)
=arctan[gt/(V0/2-gt)]