Perpendicular Lines
Perpendicular lines are lines, segments or rays that intersect to form right angles.
The symbol 
means is perpendicular to.
In the figure,
The right angle symbol in the figure indicates that the lines are perpendicular.
In three dimensions, you can have three lines which are mutually perpendicular.
Example :
By the Angle Addition Postulate,
Let x° be the measure of 
.
Then,
35° + x° = 90°.
Subtract 35° from each side.
x° = 55°
Therefore,
