S = (1/2) ∳ rr dθ = 3π/2,
x_G = (2/3S) ∳ x・dS
　　= (2/3S) ∳ x・rr cosθ dθ
　　= (1/3S) ∳ r^3 cosθ dθ
　　= 5/6,
y_G = 0,
　G(5/6, 0)