Let the normal vector of surface α be N 1 and the normal vector of surface β be N2.

Take the direction vectors A, B, C, D whose module length is not 0 on straight lines A, B, C, D respectively.

∫a∨c，b∨d .

∴A=nC,B=mD,n、m≠0.①

And N 1⊥A, b, n 1 a = 0, n 1 b = 0.

Substituting ① into ② gives NN 1 c = 0 and Mn 1 d = 0.

∴N 1⊥C and n 1 ⊥ d

Therefore, N 1 is also the normal vector of plane β, n 1∑N2.

∴ surface α ∑ surface β.