（1/2)gt^2-(1/2)g(t-1)^2=(7/16)*（1/2）gt^2
t^2-(t-1)^2=(7/16)t^2
(9/16)t^2-(t-1)^2=0
(3t/4)^2-(t-1)^2=0
(3t/4-t+1)(3t/4+t-1)=0
t1=4t2=4/7