I have a question about high school geometry proofs and I'm not quite sure how to ask it. I have been given the definition of a straight line as an angle measuring 180 degrees.
Let us say that I have angles $$\alpha, \beta$$
Such that they form a horizontal line intersected by a vertical one (where alpha and beta would be 90 degrees each in, let's say, quadrants I and II. I would like to say that $$\alpha + \beta$$ is a straight line. Is this a hypothesis or a given or is there some axiom that I have to use?
If it is given that $\alpha=\beta=90°$ and they are adjacent, you can use part and whole postulate to state that the resulting angle is $180°$. Hence, by definition, it's a straight line.