The angle of 1 ' (minute of arc) in radian is nearly equal to

First, convert one minute to a second, ${1}^{'} = \left(\dfrac{1}{60}\right) °$ and then convert it into radian, $\dfrac{1}{60} \times \dfrac{\pi}{180}rad$, so we can say that in one minute there is, $2 . 91 \times {10}^{- 4}rad$.