* Đáp án
* Hướng dẫn giải
\({\tau _x} = {\tau _c} = \frac{{2T}}{{0,7.k.\pi .{d^2}}};{\tau _u} = \frac{{{M_u}}}{{{W_u}}} = \frac{{4{M_u}}}{{0,7.k.\pi .{d^2}}};\tau = \sqrt {\tau _c^2 + \tau _u^2} = \sqrt {\tau _c^2 + 0} = {\tau _c} \le \left[ \tau \right] = \left[ {{\tau _c}} \right]\)