第 15 題
z = x + yi,-1 < x < 0,0 < y < 1
|z| = 1,x^2 + y^2 = 1
z^3 + z^2 + z + 1 = (z^2 + 1)(z + 1)
| z^2 + 1 |^2 = | (x^2 - y^2 + 1) + (2xy)i |^2 = (x^2 - y^2 + 1)^2 + (2xy)^2 = 4x^2
| z + 1 |^2 = | (x + 1) + (y)i |^2 = (x + 1)^2 + y^2 = 2x + 2
|z^3 + z^2 + z + 1| = √[4x^2 * (2x + 2)] = 2√2 * √(x^3 + x^2) ≦ 2√2 * √(4/27) = (4/9)√6