Create a Perpendicular Line Through a Point on the Line
To construct a line perpendicular to a given line through a point on that given line, we first create a line segment that has the given point as the midpoint.
1) Using the Circle with Center through Point tool 
, create a circle with a center at P and any radius.
2) Mark the points where the circle intersections the line using the Intersect tool
.
3) Now you can use what you know to create a perpendicular line through the original point, which is the midpoint between the two intersection points.
4) Verify that the new line is perpendicular to the original by using the Angle Measure tool
.
, create a circle with a center at P and any radius.
2) Mark the points where the circle intersections the line using the Intersect tool.
3) Now you can use what you know to create a perpendicular line through the original point, which is the midpoint between the two intersection points.
4) Verify that the new line is perpendicular to the original by using the Angle Measure tool.