Video:
Hypothosis, Demo of the why, and one practice probelm
https://drive.google.com/open?id=1Nw_eFaiwaZwxbOjW0UGYP4dWaVKbhbv
Click here to see an awesome video 🙂
https://www.mathsisfun.com/proof180deg.html
Practice finding the missing angle here:
Triangles
A triangle has three sides and three angles 
SIDES can be measured in inches, feet, centimeters, etc. 

The three angles always add to 180° 
ANGLES are ALWAYS measured in DEGREES ° 
Equilateral, Isosceles and Scalene, what are the SIDE LENGTHS?
There are three special names given to triangles depending on its sides.
These names that tell you how many sides (or angles) are EQUAL.
There can be 3, 2 or no equal sides/angles:
Equilateral Triangle (EQUAL)Three equal sides 

Isosceles Triangle (Interesting)Two equal sides 

Scalene Triangle (SCARY)No equal sides 
What Type of Angle?
Triangles can also have names that tell you what type of angle is inside:
Acute Triangle (Cute and Small)All angles are less than 90° 

Right Triangle (Just Right!)Has a right angle (90°) 

Obtuse Triangle (Oh wow that’s big)Has an angle more than 90° 
Combining the Names
Sometimes a triangle will have two names, for example:
Right Isosceles TriangleHas a right angle (90°), and also two equal sides and two equal angles. Can you guess the measurement of the two equal angles? 
More information and fun at https://www.mathsisfun.com/triangle.html