2 intersecting lines, one of which is vertical and one of which is horizontal. The intersection forms…

Illustration of a circle with a right angle inscribed in a semicircle.

Every elliptical involution has the property that there are two definite points in the plane from which…

Circles should be unshaded or shaded evenly with thick and thin lines, changing at about 45 degrees.

Illustration of perpendicular lines. Intersecting lines that meet at a right angle.

Illustration of perpendicular lines with the horizontal and vertical lines labeled. "A horizontal line…

A perpendicular line drawn from point P to straight line RS.

CD is the projection of AB upon OX. In each case AE=CD and AE=ABcos theta.