Answer
squaring; $x^2+y^2$
Work Step by Step
Please note that according to the definition of the trigonometric functions and by applying the Pythagorean theorem, we have the following relations between polar and rectangular coordinates:$$x=r\cos \theta, \qquad y=r\sin \theta, \\ r^2=x^2+y^2, \qquad \tan \theta =\frac{y}{x}.$$
Thus, the answers are:
squaring; $x^2+y^2$
