Answer:
Assume ΔABC is an acute triangle.
Explanation:
Given the triangle ABC.
We have to tell the first step of indirect proving ΔABC is not an acute triangle.
An indirect proof, also known a proof by contradiction. When we use indirect proof method, we assume the opposite of our theory to be true and then by applying our assumed theory we contradict the given statement which proves our supposition is wrong. In other words, we assume our theory is false.
Hence, our first step is to
Assume ΔABC is an acute triangle.
Third option is correct.