此时不重要。总为零。
一个物体若要在某一点处处于转动平衡，在这一点处的总力矩必须为零。
For an object to be in rotational equilibrium about a certain point, the total torque about this point must be zero.
实际上，这就是把负电阻接入电路，使电路的总电阻为零。
In effect, a negative resistance is inserted in the circuit so that its total resistance is zero.
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