### Теория:

Let's see the remarkable property that connects three angles of a triangle.
The sum of the measure of three angles of a triangle is 180°.
Example:
Consider a triangle $$ABC$$ with interior angles measures $$∠1$$, $$∠2$$ and $$∠3$$.  Draw a line $$DE$$ parallel to $$BC$$.

Now the angle formed by the parallel line $$DE$$ with the triangle $$ABC$$ is $$∠4$$ and $$∠5$$.

Since $$DE$$ is  parallel to $$BC$$, using the alternate interior angle property $$∠2$$ must equal to $$∠4$$.

Similarly, $$∠3$$ must be  equal to $$∠5$$.

That is $$∠2 = ∠4$$ and  $$∠3 =∠5$$.

As $$DE$$ is a straight line, $$∠5$$ and $$∠CAD$$ are linear pairs (Pair of adjacent supplementary angles).

$$∠5 + ∠CAD = 180°$$

That is, $$∠5 + ∠1 + ∠4 = 180°$$

Equivalently, $$∠1 + ∠2 + ∠3 = 180°$$.

It states that the total measures of the three angles of a triangle is $$180°$$.