[填空题]
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A particle P moves in a straight line 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.
2. Hence or otherwise, find all possible values of t for which the velocity of P is increasing.$a \lt t \lt b $ a = b =
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.
