Pythagorean Theorem

This is one of the most fundamental concepts in math, and as a result it is something that will be applicable on every single ACT. The pythagorean theorem applies to right triangles and states that the square of the hypotenuse is equal to the sum of the squares of the other sides.

A question may simply show a triangle directly and ask for either the side lengths or a calculation that requires knowing all of the side lengths (such as perimeter). This may also be usable in a word problem.

Be on the lookout for if a word problem explicitly states that something happens at a 90-degree angle or if there is a perpendicular line or tangent line. These all lead to 90 degree angles and mean that a right triangle may be created, and so the pythagorean theorem is valid.