While there are a great number of math formulas that students need to memorize for ACT day (you can find them in my cheat sheets here), I want to remind students and parents of 3 that you do not need to know. If you happen to know them or have them memorized, then they could come in handy. However, they are either unnecessary or have workarounds that are simpler than their memorization.
The first is the Quadratic Formula, which you can see here. The Quadratic Formula is used to solve a quadratic, (taking the form ax^2 + bx + c = 0). A quadratic takes the form of a parabola (when graphed). A parabola's solutions are where it crosses the x-axis; sometimes a quadratic has two solutions, sometimes one (if it's vertex is tangential with the x-axis), and sometimes zero (if it never crosses the x-axis at all).
However, the quadratic formula is reserved for the most difficult quadratics. Never have I seen on a previous ACT test a quadratic or trinomial that could not be solved using simple factorization and setting each factor equal to zero: don't spend time trying to memorize the quadratic formula!
The second is the distance formula, which you can also see here. The distance formula is used to find the distance between two points on the (x, y) coordinate plane. However, if you do not have this formula memorized already, think about doing this instead: plot your two points on a graph, and pretend that the distance between them is they hypotenuse of a right triangle. You will know the lengths of the other two sides based on the graph, and can then use The Pythagorean Theorem to find the triangle's missing side (aka the distance between the two points).
The third equation you do not need to memorize is the volume of any asymmetric shape, such as a sphere or a cone. Historically, if the ACT asks you to use or find the volume of such a shape, you will be given the equation.
I have seen, however, previous ACT math questions that require that you do know the formula for the volume of a shape that is symmetric (such as a cylinder or a cube). In such cases, the volume is the area of one end (in the case of a cylinder that would be the area of the circle) times the height.
All of these and more are covered in depth in The ACT System course!