If we have a quadratic equation described in a table and it does not show the exact solution (roots) of the equation, we can look if, with the values of x or the independent variable sorted, we have a change of sign.
This indicates that there is a root between those two values of x.
For example:
x = 2 --> f(x) = -3
x = 3 --> f(x) = 4
We can see that from x=2 to x=3, we have a sign change. Then we know that, because of the continuity of the quadratic function, we must have a value between x=2 and x=3 for which f(x)=0. This is an application of the Intermediate Value Theorem.
We can then approximate the value of the root x=r as the average between x=2 and x=3. This is the bisection method to find roots of functions. In this case, it would give a result r=2.5.
There are other methods (Newton-Raphson or False position, for example), but this bisection method is the simplest approximation.
