Q15 (c)

A sound wave can be modelled using a function $P(t)=k\sin(at)$, where $P$ is air pressure in Pascals, $t$ is time in milliseconds (ms), and $k$ and $a$ are constants.

Hence, find the values of $t$, where $0<t<10$, for which functions $P_1(t)$ and $P_2(t)$ are BOTH decreasing.