(Derivation and principal-value conversion)
(a) Starting with the definition of one radian, derive the general relation connecting angle measures in degrees and radians.
(b) Using the relation obtained in part (a), convert an angle of −525∘ into radians and express the answer as an exact multiple of π.
(c) Reduce the radian measure obtained in part (b) to its principal value lying between −π and π.