A,V^2=2gh,h=V^2/2g
B.由h=(1/2)gt^2-(1/2)g(t-10)^2 求出t,H=(1/2)gt^2
c.(1/2)gT^2-(1/2)g(T-t)^2=1,已知t,由此可求出T,H=(1/2)gT^2