Chapter 2.6, Problem 5ST2

To determine

To state: the theorem or the definition that justifies the given statement.

Explanation of Solution

Given information:

The given statement is “ $\angle 8$ is a $90$ degrees angle”

And the given figure is as follows.

  $\overline{DE}\perp n$

Consider $\overline{DE}\perp n$

As per the definition of perpendicular lines, the perpendicular lines intersect to form a

right angle.

So, the $\overline{DE}$ forms $90$ degrees angle with line n.

Thus, $\angle 8$ is a $90$ degrees angle

Hence, the definition that justifies the given statement is perpendicular lines.