Line SA passes through the vertex of the square ABCD, with SA being perpendicular to AD.

Line SA passes through the vertex of the square ABCD, with SA being perpendicular to AD. Prove that the plane of the square is perpendicular to the plane SAB.

We proceed from the fact that the vertices of the square are indicated by dots clockwise, starting from the lower left corner. Then the line SA drawn through point A will also be perpendicular to the segment AB, which is the side of the square perpendicular to the side AD. Thus, we have 3 points S, A and B, through which we can draw a plane and this plane, in any case, will be perpendicular to the plane in which the square ABCD is located, just purely from the construction.