设总的运动时间为ts总=1/2at^2s（t-1）=1/2a(t-1)^2s总-s（t-1）=1/2a（2t-1）[s总-s（t-1）]/s=（2t-1）/t^2=7/16（t-4)(7t-4)=0所以t=4或7/4
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