고체역학 (Beer) — Torsion: Problem 3.21

3.21 Two solid steel shafts are connected by the gears shown. A torque of magnitude \( T = 900 \text{ N}\cdot\text{m} \) is applied to shaft \( AB \). Knowing that the allowable shearing stress is \( 50 \text{ MPa} \) and considering only stresses due to twisting, determine the required diameter of (a) shaft \( AB \), (b) shaft \( CD \).