I like the idea of folding the triangle down so that all angles are in a straight line to be useful.

Today the main problem is:

# Types of angles

**Acute angle:**

An angle whose measure is less than 90 degrees. The following is an acute angle.

**Right angle:**

An angle whose measure is 90 degrees. The following is a right angle.

**Obtuse angle:**

An angle whose measure is bigger than 90 degrees but less than 180 degrees. Thus, it is between 90 degrees and 180 degrees. The following is an obtuse angle.

**Straight angle**

An angle whose measure is 180 degrees.Thus, a straight angle look like a straight line. The following is a straight angle.

**Reflex angle:**

An angle whose measure is bigger than 180 degrees but less than 360 degrees.The following is a reflex angle.

**Adjacent angles:**

Angle with a common vertex and one common side. <1 and <2, are adjacent angles.

**Complementary angles:**

Two angles whose measures add to 90 degrees. Angle 1 and angle 2 are complementary angles because together they form a right angle.

Note that angle 1 and angle 2 do not have to be adjacent to be complementary as long as they add up to 90 degrees

**Supplementary angles:**

Two angles whose measures add to 180 degrees. The following are supplementary angles.

**Vertical angles:**

Angles that have a common vertex and whose sides are formed by the same lines. The following(angle 1 and angle 2) are vertical angles.

When two parallel lines are crossed by a third line(Transversal), 8 angles are formed. Take a look at the following figure

Angles 3,4,5,8 are interior angles

Angles 1,2,6,7 are exterior angles

**Alternate interior angles:**

Pairs of interior angles on opposite sides of the transversal.

For instance, angle 3 and angle 5 are alternate interior angles. Angle 4 and angle 8 are also alternate interior angles.

**Alternate exterior angles:**

Pairs of exterior angles on opposite sides of the transversal.

Angle 2 and angle 7 are alternate exterior angles.

**Corresponding angles:**

Pairs of angles that are in similar positions.

Angle 3 and angle 2 are corresponding angles.

Angle 5 and angle 7 are corresponding angles

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