The polar coordinates of the Cartesian point $\left(x\,y\right)\=\left(1\,2\right)$ are

$r\=\sqrt{x^2\+y^2}\=\sqrt{1^2\+2^2}\=\sqrt{5}$ and $\mathrm{\θ}\=\arctan \left(y\,x\right)\=\arctan \left(2\,1\right)\=\arctan \left(2\right)$ 

Hence, $\left(r\,\mathrm{\θ}\right)\=\left(\sqrt{5}\,\arctan \left(2\right)\right)$. Note the use of the two-argument arctangent function, which returns an angle in one of the four quadrants. Although it is easier to use for normal manipulations, the one-argument arctangent function, namely, $\arctan \left(y\/x\right)$ returns an angle in either the first or fourth quadrants.