Fun with Functions

We will graph different sets of functions together on the same axes and investigate the relationships between the graphs.
 

1.  Each of these equations are of the form y = x + c, where c is an integer ranging from -4 to 4. All eight of these lines are parallel, having slope value 1. They differ by their position in the coordinate plane, given by their y-intercepts. The value of c determines the y -intercept. The value of c could also be described as the number of units each graph is shifted vertically from the base graph y = x. A second perspective reveals the value of c is the opposite (or negative) of the x-intercept of the graph. From this perspective the value of c could also be described as the negative of the number of units each graph is shifted horizontally from the base graph y = x. (Note that y = x+c = (x - (-c) ) + 0.)
     

    2.  Each of these equations is of the form y = (x-2) + c where c is an integer ranging from -4 to 4. Again the lines are parallel with slope 1. The integer c plays exactly the same role in vertical or horizontal shifting as in problem 1. The set of #2 graphs can be seen as shift of the set of #1 graphs +2 units horizontally, with y = x-2 playing the role of the base graph that y = x played above. Alternately, each of the #2 graphs can be seen as a vertical shift by -2 units from the corresponding #1 graph. (Note that y = (x-2)+c = x + (c-2) = (x+c) -2.)


    3.  Each of these equations is of the form y = c (x-2) where c is an integer ranging from -4 to 4. All eight of the lines are concurrent at the point (2,0). None of the lines are parallel. The value of c determines the slope of the line. As c ranges from -4 to 0 to 4, the slopes of the resulting lines range from large negative to 0 to large positive. As a result, the line behavior ranges from rapidly decreasing from left to right to remaining constant at y=0 to rapidly increasing from left to right. Each graph y = c(x-2) represents a rotation of the base graph y=1(x-2) about the key point (2,0).


    4. Each of these equations is of the form y = x^2 + c, where c is an integer ranging from -3 to 3. The value of c is number of units that each graph is shifted from the base parabola y=x^2. The result is a set of nested, non-intersecting parabolas with vertices on the y-axis at (0,c).


    5.  Each of these equations can be written in the form y = (x - c)^2. For example y = the c value for (x+3)^2 is -3. The range of c values is integers from -3 to 3. The value of c represents the number of units each graph is shifted horizontally from the base graph parabola y = (x+0)^2. The result is set of overlapping parabolas with vertices on the x-axis at (c,0).


Classroom Application
For students who are just beginning to investigate the concept of shifting graphs of functions with the addition or subtraction of a constant, I think using graphs in pairs or threes by comparing each to the base graph is the best approach.  This activity would allow students to recognize the graphical effect of a single algebraic change and later generalize to a pattern.For exampe, first make sure students are comfortable with the behavior of the base parabola y=x^2.  Then ask them to predict the behavior and location of y = x^2 + 1. Graph the equation to test their prediction. As soon as they begin to establish  the connection between adding a constant and a vertical shift, ask them to produce an equation for a particular shift, say 4 units down.  If they can do this, then it is appropriate to simultaneously graph a set of equations y = x^2 +c so that students can solidify their generalization.  A set of several graphs is most useful and meaningful after comparisons of individual graph have introduced the concept.