Determine the domain of the function. This helps to exclude values that are of no consequence to the proportionality of the mathematical rule in question. For example, to investigate the square root function, you would set the investigative domain to all positive real numbers, due to negative numbers not having square roots in standard mathematics.
Solve for the zeros of the equation. This process involves factoring the equation and setting all factors equal to zero.
Graph the function, by hand or with the aid of a graphing calculator, between the lowest value and highest value zero point on the number line. A proportional graph has ever-increasing values between these two points.