Calculate the first three instants in time $($t $>0)$ for which the following functions are zero, by first converting to a single sinusoid: $($a$)$ cos $3$t $7$ sin $3$t; $($b$)$ cos $(10$t $+45)$; $($c$)$ cos $5$t$$ sin $5$t; $($d$)$ cos $2$t $+$ sin $2$t$$ cos $5$t $+$ sin $5$t