Answer
$$r= \sqrt 2$$
Work Step by Step
As we are given that $$x^2+y^2=2$$
To convert this equation into polar equation, we will substitute $x=rcos\theta$ and $y=rsin \theta$
$$r^2cos^2\theta + r^2sin ^2\theta =2$$
Thus, $$r^2=2$$
or $$r= \sqrt 2$$
