物体水平移动距离 X = V0 * t = L * cosα垂直方向移动距离Y = (1/2)gt^2 = L * sinα第一个式子两端平方V0^2 * t^2 = L^2 * (cosα)^2与第二个式子做比值运算 消 t^22 * V0^2/g = L * (cosα)^2/sinαV0^2 = gL(cosα)^2/(2sinα) = (10 * 75 /2)(cosα)^2/sinαV0 = 5√15 * cosα/√sinα