Example 1 As shown in the figure, intersects the sides of at points and , points and , points and , respectively. Given that the perpendiculars from points , , to , , intersect at point . Prove: The perpendiculars from points , , to , , also intersect at one point.
Solution
Proof: Taking point as the origin and as the -axis to establish a Cartesian coordinate system. Let the line passing through point and perpendicular to be denoted as . The line passing through point and perpendicular to intersects at point . Clearly, the two lines passing through points and and perpendicular to are symmetric about the -axis; similarly, the two lines passing through points and and perpendicular to are symmetric about the line passing through point and perpendicular to ; the same applies to points and .
Let the equation of line be ,
and the equation of line be .
Then the equation of line is ,
and the equation of line is .
Therefore, the family of lines passing through point is given by
and is one of these lines.
Let the equation of line be
Thus, the equation of the line passing through point and perpendicular to is
The family of lines passing through point is given by
When and , the above equation is obtained, meaning that the line passing through point and perpendicular to also passes through point .
Therefore, the perpendiculars drawn from points , , and to , , and respectively intersect at a single point .