Question

已知一物体运动的位移S与时间t之间的函数关系式S(t)=1/3t3-5t2+20t-1,则该物体运动速度不超过4m/s的时间有

Answer 1

S(t)=1/3t3-5t2+20t-1
S`(t)=V(t)=t^2-10t+20≤4
t^2-10t+16≤0
(t-2)(t-8)≤0
2≤t≤8

Answer 2

任务中~闪过~

Answer 3

v=ds/dt=t^2-10t+20=(t-5)^2-5<=4
(t-5)^2<=9
-3=<t-5<=3
2=<t<=8