1 h/2=gt^2/2
h=gt^2
gt^2=gt全^2/2
t全=2t^2 开根号
2 v^2=2gh
h=v^2/2g
h/2=v^2/4g
v^2/4g=gt^2/2
gt^2=v^2/2g
t=v^2/2g^2 开根号