The Product of two Linear Functions Gives a Quadratic Function


The Product of two Linear Functions Gives a Quadratic Function


The product of two linear functions gives a quadratic function. This property is explored interactively using an applet.

Let h and g be two linear functions of the form

h(x) = a x + b

and
g(x) = A x + B

where a and A are non zero constants. It can easily be shown that the product of functions h and g is a quadratic function. Let f be the function obtained as the product of g and h as follows:

f(x) = (h . g) (x) = h(x) . g(x) = ( a x + b ) . ( A x + B )

= a . A . x 2 + (a . B + b . A) x + b . B

An applet below may be used to explore the properties of the quadratic function f obtained above by changing the parameters a, b , A and B included in the definition of the two linear functions. There are other tutorials you may want to work through later:
tutorials on quadratic functions and graphing quadratic functions.

A - Quadratic Functions From Linear Functions : Interactive Tutorial


The button below starts the applet on a separate large screen.

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  • Click on the button above "click here to start" to start the applet and maximize the window obtained.

  • By default coefficients a, b, A and B are set as follows: a = 1, b = 2, A = 1 and B = 0. Explain, graphically, why the product of the two linear functions gives a function that increases indefinitely on the left side and right side.

  • Change coefficient A to -1. Explain, graphically, why the product of the two linear functions gives a function that decreases indefinitely on the left side and right sides.

  • Change all four coefficients and note that the x intercepts of the parabola are the x intercepts of the two lines. Explain.


  • Change all four coefficients and note that the x coordinate of the vertex of the parabola is the average of the x coordinates of the x intercepts of the parabola. Explain.

  • Set a, b, A and B such that: A = k a and B = k b. For example a = 1, b = 2, A = 2 a = 2 and B = 2 b = 4. The parabola has only one x intercept. Explain.

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