Friction on two inclined planes. In Fig. $3.23$ planes $\mathrm{l}$ and 2 are both rough with coefficients of friction $\mu_{1}$ and $\mu_{2} .$ Find the relation between $M_{1}, M_{2}, \theta_{1}, \theta_{2}, \mu_{1}$, and $\mu_{2}$ such that
(a) $M_{1}$ is about to slip down plane 1 .
(b) $M_{2}$ is about to slip down plane 2 .