ACT Lines and Angles

ACT Math: The Basics of Lines & Angles

Need better scores in Geometry? This quick review will tell you everything you need to know about Geometry basics! These are the essential building blocks for ACT Math success.

An angle is formed by two lines or line segments which intersect at one point. The point of intersection is called the vertex. Angles are measured in either degrees or radians.

A circle has 360 degrees total. You might see in your online studying questions involving radians, and some test questions will ask you to convert radians to degrees. To convert from degrees to radians, multiply by π/180.  To convert from radians to degrees multiply by 180/π.

An acute angle is an angle whose measurement in degrees is between 0 and 90. A right angle is an angle whose measurement in degrees is exactly 90. An obtuse angle is an angle whose degree measure is between 90 and 180. A straight angle is an angle whose degree measure is exactly 180 degrees.

All of the angles on one side of a straight line sum to 180 degrees.
a + b + c = 180 degrees


All of the angles around one point must sum to 360 degrees.
a + b + c + d + e = 360 degrees




Perpendicular lines are formed when the angle between two lines is 90 degrees. The shortest distance from a point to a line is a line with a length such that the two lines form a 90 degree angle.

Two angles are supplementary if they share one line; i.e., if they sum of their angles is 180 degrees. Two angles are complementary if together they make a right angle; i.e., if the sum of their angles is 90 degrees.

To bisect an angle means to cut it in half. The two smaller angles will then have the same measurement.

If two parallel lines intersect with a third line, the third line is called a transversal. When this happens, all acute angles are equal and all obtuse angles are equal. Each acute angle is supplemental to each obtuse angle.

x and y are parallel lines, and z is the transversal
a = d = e = h
c = b = g = f

Vertical angles are a pair of opposite angles formed by intersecting lines. For the figure, a and d is an example of a pair of vertical angles. Vertical angles are equal.