[填空题]
A particle P moves so (jqkd- 7,cjaaa m3wv2 that u4:c78 wby;euevugg7its velocity, in m s$^{−1}$ , at time t seconds can be described by the function
v(t)=2cost, where t≥0. The kinetic energy of the particle, in joules (J), is given by the function E(v)=3v$^2$ .
1.Find an expression for E as a function of time.
E(t)=a cos$^2$ t; a= .
2.Hence, find E'(t)=-a cost sint; a= .E'(t)=
3.Hence or otherwise, find the first time at which the kinetic energy is changing at a rate of 4 J s$^{−1}$.
t= s
