Common Misconceptions

Misconception 1: "A cubic graph always looks like an S-shape"

The basic y = x³ has a pure S-shape, but most cubic graphs have a hill and valley shape with a local maximum and local minimum. This is because the additional x² and x terms shift and tilt the curve, creating two turning points. Only simple cubics like y = x³ or y = x³ + k have the smooth S-shape with no turning points. When sketching, always check whether the cubic has 0 or 2 turning points before drawing.

Misconception 2: "A cubic can only cross the x-axis once"

A cubic can cross the x-axis 1, 2, or 3 times depending on its equation. A quadratic can have 0, 1, or 2 roots; a cubic always has at least 1 root but can have up to 3. The number of visible crossings depends on where the turning points sit relative to the x-axis. If both the local max and local min are above the x-axis, the curve only crosses once; if one is above and one below, it crosses three times.

Misconception 3: "At a repeated root, the cubic crosses the x-axis like normal"

At a repeated root, the cubic graph TOUCHES the x-axis but does NOT cross through it — the curve bounces off like a ball hitting the floor. This is because the factor appears squared: y = (x − 2)²(x + 1) touches the x-axis at x = 2 and crosses it at x = −1. The difference between touching and crossing is a key feature examiners test in sketch questions.