Finding the Current I1 using Polar Notation
Starting from here,
$$I_1 = \frac{1-100j}{10001}\times 5\angle 0$$

Converting this to polar form, the magnitude is given by
$$|I_1| = \frac{\sqrt{10001}}{10001}\times5 = 0.04999750$$ 
the phase angle is given by
$$\angle I_1 = \arctan(-100) = -1.56 \ \mathrm{rad} = -89.4\ \mathrm{degrees}$$