1. If a straight line is perpendicular to two intersecting straight lines on a plane, it is perpendicular to the surface. Two parallel lines, one perpendicular to the plane and the other perpendicular to the plane. 3. If two planes are perpendicular, the straight line intersecting vertically in one plane is perpendicular to the other plane. 4. Vector method. That is, if the vector product is zero, two vectors are vertical to prove that the straight line is vertical, and then it is proved by 1. (Vector method is generally not used to prove that a straight line is perpendicular to a plane, but is mostly used to find dihedral angle, included angle between a straight line and a plane, etc. )