What is Radian?

The arc length is $s = r\theta$ when $\theta$ is in radians. To convert, $\pi\ \text{rad} = 180^\circ$, so $1\ \text{rad} = \dfrac{180^\circ}{\pi} \approx 57.3^\circ$. Calculus and physics use radians, because the formulas such as $\dfrac{d}{dx}\sin x = \cos x$ are simple only when the angle is in radians. Make sure your calculator is set to the right mode.