0.25(t的平方)-5t=200 t²-20t-800=0 (t-40)(t+20)=0 t1=40,t2=-20,3,t^2-20t-800=0 (t-40)(t+20)=0 t1=40,t2=-20,2,0.25t²-5t=200 0.25t²-5t-200=0 t²-20t-800=0 (t+20)(t-40)=0 t1=-20，t2=40,1,
