斜抛运动y=v0.sinα.t-gt^2/2x=v0.cosα.tvy=v0.sinα-gt=0时达最高点，t1=v0.sinα/g达地面时间t=2.t1=2v0.sinα/gt代入xX=v0.cosα.2v0.sinα/g=2v0^2(.sin2α/2g)=v0^2.sin2α/g可见，sin2α=1，α=π/4--->Xmax=v0^2/g
