At least one angle is acute.

What is the first step of indirectly proving this statement?

Assume at least one angle is not acute.

Assume none of the angles are acute.

Assume none of the angles are obtuse.

Assume at least one angle is obtuse.

Acute angles are those angles that measure less than 90 degrees while obtuse angles have measurements greater than 90 degrees. In this case, to prove that at least one angle is acute, then one angle has to be obtuse. This is to supplement/complement the large measurement and make up a closed polygon still.