力矩.M=「=ol=0(2) M2=|yo=0(3) E 由()N=E2l E vA==0 中性轴为形心轴 由(2)M,=「 E E zy-dA=-l,=0 KDD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  