力矩.M,=zo=0(2) M=you=0(3) 由(1)N=[E E E dA=-S.=0 中性轴为形心轴 由(2)M 1y-/b E =0力矩. = = 0 A M y zdA (2) = = 0 A Mz ydA (3) 由(1) = = = = 0   z a A S E ydA E dA y N E    中性轴为形心轴 由(2) = = = 0  yz A y I E dA E M zy  