in a diagram <a and <b are vertical
In a diagram, "<a" and "<b" being vertical means that the angles labelled "a" and "b" are formed by two intersecting lines or line segments, and these angles are opposite each other. They are also referred to as vertically opposite angles.
Here is an example of how <a and <b can be vertical in a diagram:
|\
| \
a | \ b
| \
________|____\______
| \
In this example, the lines intersect at a point, and angles "a" and "b" are formed opposite each other. Thus, they are vertical angles.