1)x1=at^2/2
x2=a(2t)^2/2
x2-x1=6
即a(2t)^2/2-a(t)^2/2=6得a=4m/s^2
x5=at^2/2=4*25/2=50m
2)v^2-v0^2=2ax
x=(v^2-v0^2)/2a=(100-36)/8=8m