Step 1: link AB

Step 2: Find the midpoint O of the line segment AB.

Step 3: Do OP⊥AB, where O is the midpoint of AB and P is on L.

Principle: If P is outside AB, then OP⊥AB, "In the line segment between a point outside the straight line and each point of the straight line, the vertical line segment is the shortest".

Question 1:

Step 1: Connect AB, and intersect with straight line L at point P.

Principle: P is on AB, "the line segment between two points is the shortest"

Question 2:

Step 1:

Link AB

Step 2: Find the midpoint O of the line segment AB.

Step 3: make OP⊥AB, where O is the midpoint of AB and P is on MN.

Principle: If P is outside AB, then OP⊥AB, "In the line segment between a point outside the straight line and each point of the straight line, the vertical line segment is the shortest".