**Supplementary Angles**
You know that a straight angle is 180°. Therefore we can say that:
**Λb + Λx = 180°**
You also know that the angle sum for a triangle is 180°.
**Λa + Λb + Λc = 180°**
According to the transitive law (Operator and Calculation Rules 8.5):
**Λb + Λx = Λa + Λb + Λc**
You subtract the angle b on both sides:
**Λx = Λa + Λc**
Then we can see that the outer angle of the triangle is equal to the sum of the two opposite angles in the triangle:
**Λa + Λc = Λx** |