Do you always set a cubic and quadratic function to zero?

Answer 1

We commonly set quadratic, cubic, and even linear functions equal to zero to find their roots, otherwise known as the zeroes of a function! These (if they are real-valued and unique) are values that intercept the x-axis.

There are cases where you wouldn't want to set those types of functions equal to zero (differential equations, advanced calculus, etc.) but it all depends on context.

Answer 2

You would set them equal to zero to find the zero's of the functions.