钢杆的圆截面上作用有轴力N，弯矩My，扭矩T。若已知许用应力[σ]，截面积A，抗弯截面系数W，正确的强度条件为______。 A =dfrac (N)(A)+dfrac ({M)_(y)}(W)leqslant [ 0] . =dfrac (T)(2pi )leqslant [ t] =dfrac ([ O] )(2);-|||-_(2)(A)_(1)+dfrac ({M)_(y)}({N)_(y)}sqrt ({{M)_(y)}^2+({M)_(2)}^2}leqslant [ 0] ;-|||-=sqrt ({(dfrac {N)(A)+dfrac (My)(pi ))}^2+((dfrac {T)(pi ))}^2}leqslant [ 0] ;-|||-D sqrt ({(dfrac {N)(A))}^2+((dfrac {{M)_(3)}(N))}^2+((dfrac {T)(N))}^2}leqslant [ 0]