# If two lines are perpendicular, then each of the adjacent angles

- In the figure above, lines l
_{1} and l_{2} intersect at point O, and they are perpendicular to each other,
then four angles are formed, angle1, angle2, angle3 and angle4. For angle1, its adjacent angles are angle2 and angle4.
Angle1 and angle2 are an pair of adjacent angles. Angle1 and angle4 are another pair of adjacent angles.

- What property does the adjacent angles have?
- Therefore, The sum of an adjacent angles pair is 180 degrees.
- Adjacent angles are equal and each equals to 90 degrees.