The first step of an indirect proof is to assume the opposite, or negation, of the given statement.
Assume temporarily that no angles are obtuse.
Practice makes perfect
The first step of an indirect proof is to assume the opposite, or negation, of the given statement. Let's think about the given statement.
At least one angle is obtuse.
To write a negation of the given statement, we need to consider how to say its opposite. Here we can do this by changing at least one into no.
Negation: & No angles are obtuse.
Since the first step is to temporarily assume the opposite, we should include this fact in our sentence.
First step:
Assume temporarily that no angles are obtuse.
