$3-$ A pair of gears, the wheel has a radius of Addendum is $2$Rcos$()$ and the pinion has a radius of Addendum is $2rcos().$ The equation of the Arc of the path contact is $...........,$ where $$ is the pressure angle

$($a$)(R-r)[1\mathrm{.}73+tan()]$

$($b$)(R+r)[1\mathrm{.}73-tan()]$

$($c$)(R-r)[1\mathrm{.}73-tan()]$

$($d$)(R+r)[1\mathrm{.}73$ftan$()]$