N=|odA=0→|E-dA=0→yda=0 S.=0中性轴过形心 M,=∫zadA=0→z:E-dA=0→/=0 M=[y.dA=M→[y.E8A=M EL My ENx A A = =  d 0  =  E y A A  d 0  =  y A A d 0 Sz = 0 中性轴过形心 My z A A =  =  d 0   =  z E y A A  d 0  I yz = 0 Mz y A M A =  =  d   =  y E y A M A  d 1  = M EIz  =  E y A M A  2  d  = E = y M y I z   = E y