The beeline which is perpendicular to middle plane before deformation is still a beeline after deformation and is still perpendicular to the bent middle plane. Namely Yx=O y 0 (3) the board surface is neutrosphere Na amely )20=0,()2。=0 From the geometric functions =0)=0)=0 (4) the stress 0 has very small effect on deformation, thus can be ignore ed Scilicet we think: 0.=011 The beeline which is perpendicular to middle plane before deformation is still a beeline after deformation, and is still perpendicular to the bent middle plane. Namely :  xz = 0,  yz = 0 （3）the board surface is neutrosphere Namely ( ) 0, ( ) 0 0 0 = = z= z= u v From the geometric functions: ( ) 0, ( ) 0, ( ) 0 0 0 0 = = = = = z= xy z x z y    （4）the stress has very small effect on deformation, thus can be ignored. Scilicet we think: = 0  z  z