When converting a complex number from rectangular form to polar form, "θ" must always be expressed in radians.

When converting a complex number from rectangular form (a + bi) to polar form, it is essential to express the angle "θ" in radians. This is critical because many mathematical applications and formulas, especially in calculus and engineering, require angles in radians for accuracy and consistency.

In polar form, a complex number is represented as $r(\cos \theta + i \sin \theta)$, where $r$ is the modulus (magnitude) of the complex number, and $\theta$ is the argument (angle). Radians provide a direct relationship between the arc length and radius in a circle, making calculations more straightforward and aligned with mathematical principles.

To convert the angle to radians, you can use the formula: $\theta = \tan^{-1}\left(\frac{b}{a}\right)$ where $a$ is the real part and $b$ is the imaginary part of the complex number. Radians are preferred because they simplify the representation and computation in various fields such as physics, engineering, and computer science, where complex numbers frequently arise.

In summary, when expressing the polar form of a complex number, it is essential to maintain consistency and clarity by using radians for the angle "θ".