Answer:
Assume at least one angle is not acute.
Step-by-step explanation:
With an indirect proof, instead of proving that something must be true, you prove it indirectly by showing that it cannot be false.
An indirect prove starts assuming that the statement is false and then your goal is to arrive at a contradiction. Then, in this case, the first step of indirectly proving at least one angle is acute is assume at least one angle is not acute.
