Pretty easy to see that each vertex is a/2 away from the origin in both the x-direction and the y-direction and that each is at a 45° angle.
The polar coordinates are in the form of (r, θ) where "r" is the distance from the origin. Calculate "r" using the Pythagorean Theorem:
(a/2)2 + (a/2)2 = r2
r = √(a2/4 + a2/4) = √(a2/2) = a/√2 = a√2/2
This will be the same value of "r" for all 4 vertices.
For the vertex in quadrant I, θ = π/4 which makes to polar coordinate: (a√2/2, π/4)
For the vertex in Q II, θ = 3π/4
For the vertex in Q III, θ = 5π/4
For the vertex in Q IV, θ = 7π/4