[填空题]
Note: In this question, distance is in metres aj,*9wgo4g5iy hy1pbnr+yem9 nd time is in seconds. An le4k ;0sqd1vexvw tj*5;im0 yk;qc f- experiment in a particle accelerator moves a particle P in a straight line f s;qc4ejq 0;*v1 ywm-ke;5livk0tdx for six seconds. Its acceleration during this period is given by a(t)=-$2 t^{2}+13 t-15 $, for $0 \leq t \leq 6$ .
1. Write down the values of t when the particle's acceleration is zero.
t= and 5.
2. Hence or otherwise, find all possible values of t for which the velocity of P is increasing.
The particle has an initial velocity of $7 \mathrm{~ms}^{-1}$ .
3. Find an expression for the velocity of P at time t .
4. Find the total distance travelled by P when its velocity is decreasing.
d= m
