An x-intercept has a y-value of zero, so multiplying that output by a constant still gives zero: a·0 = 0. Consequently, any input where f(x) equals zero remains an input where af(x) equals zero. Other points shift vertically according to their original output, while the locations of the intercepts on the x-axis stay fixed.
The multiplier increases the magnitude of every nonzero output. A positive y-value becomes a larger positive value, moving the point farther above the x-axis, while a negative y-value becomes more negative, moving it farther below the axis. The direction of each point relative to the x-axis remains the same, but its vertical distance increases.
The constant a determines how strongly the graph is stretched. When a is greater than 1, each output is multiplied by that same factor, so a point with output y moves to an output of ay. Larger values of a produce greater vertical distances from the x-axis, allowing equations within the same function family to be compared.
Start with selected points or output values from the original graph y = f(x). Keep every x-coordinate unchanged, multiply each corresponding y-value by a, and plot the resulting points. Connecting those points according to the original graph produces y = af(x). Checking the x-intercepts helps confirm that the transformation was applied consistently.
Compare corresponding outputs at the same input values. If the second function consistently multiplies those outputs by the same constant greater than 1, it can be represented in the form y = af(x). The unchanged x-coordinates and fixed x-intercepts provide additional graphical evidence that the difference comes from vertical stretching rather than a change in horizontal position.
It is useful when a model must represent amplified magnitudes while preserving the same input values. The transformation lets students compare related functions, examine how output size changes, and connect an equation with its graph. In applications, it can describe situations where measured quantities become larger in magnitude without changing the corresponding x-values.