Q.Define a radian.

Angles can be measured in degrees (the everyday unit) or radians (the natural unit for circular motion and calculus). One radian is the angle subtended at the centre of a circle by an arc whose length exactly equals the radius; for an arc of length $s$ on a circle of radius $r$, the subtended angle in radians is $\theta=s/r$. Since the full circumference $2\pi r$ subtends the full $360°$ turn, $360°=2\pi$ rad, so $1\text{ rad}=\dfrac{180° …