Lines in motion
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Lines in Motion. Lesson 4.3. Earlier you worked with two forms of linear equations: Intercept form : y = a+bx Point- slope form : y=y 1 +b(x-x 1 ) In this lesson you will see how these forms are related to each other graphically.

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Lines in Motion

Lesson 4.3


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  • Earlier you worked with two forms of linear equations:

    • Interceptform: y =a+bx

    • Point-slopeform: y=y1+b(x-x1)

  • In this lesson you will see how these forms are related to each other graphically.

  • With the exception of vertical lines, lines are graphs of functions. That means you could write the forms above as f(x)=a+bx and f(x)=y1+b(x-x1) .

  • The investigation will help you see the effect that moving the graph of a line has on its equation. Moving a graph horizontally or vertically is called a translation.

  • The discoveries you make about translations of lines will also apply to the graphs of other functions.


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Movin’ Around

  • In this investigation you will explore what happens to the equation of a linear function when you translate the graph of the line. You’ll then use your discoveries to interpret data. Graph the lines in each step on the same set of axes and look for patterns.

  • On graph paper, graph the line y=2x and then draw a line parallel to it, but 3 units higher. What is the equation of this new line? If f (x)=2x, what is the equation of the new line in terms of f (x)?


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Movin’ Around

  • Draw a line parallel to the line y=2x, but shifted down 4 units. What is the equation of this line? If f(x)=2x, what is the equation of the new line in terms of f (x)?


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Movin’ Around

  • Mark the point where the line y=2x passes through the origin. Plot a point right 3 units from the origin. Draw a line parallel to the original line through this point. Use the point to write an equation in point-slope form for the new line. Then write an equation for the line in terms of f (x).


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f(x)

(h,k)

g(x)=f(x-h)+k


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Movin point up k units and right h units, to create a new function g(x), what is the equation of this translated ’ Around

  • In this investigation you will explore what happens to the equation of a linear function when you translate the graph of the line. You’ll then use your discoveries to interpret data. Graph the lines in each step on the same set of axes and look for patterns.

  • On graph paper, graph the line y=2x and then draw a line parallel to it, but 3 units higher. What is the equation of this new line? If f (x)=2x, what is the equation of the new line in terms of f (x)?


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Alternative Moving Along point up k units and right h units, to create a new function g(x), what is the equation of this translated

  • Set up a course as illustrated in the figure below. Position A, B, and the start should be labeled with the correct distances between the three locations. Set up a walker who will walk at a rate of ½ meter per second.


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At 0 seconds the student begins to walk at the rate of ½ meter per second toward point A. Record the time and distance of the student from position A for the first 5 seconds.

At time 5 seconds, the student will momentarily stop and then begin walking backwards at the same rate.

For time 5 -10 seconds, record the time and distance the student is from both point A.

For the second path, have a second student again walk at ½ meter per second, but begin at time 0 seconds where the first student was at time 2 seconds on the first walk. Record the time and how far the this student is from position B.

For the first three seconds the student should walk forward for and then momentarily stop and then begin walking backwards at the same rate.

For the next 7 seconds record the time and position the student is from position B.

  • Two students will walk a similar path. Another student will call out the seconds starting at zero seconds. At each second interval, stop and record the time and position of the student.


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  • Enter the data in the graphing calculator. Use a Data and Statistics Page. Place Time A, Time B, Distance A, and Distance B each in a list.

  • Open a second Data and Statistics page and split the page so you can create two graphs by using the Tool key.

  • Create one graph for Time A vs. Distance A and a second graph for Time B vs. Distance B.

    • How are the two graphs related to each other?

    • If A’s graph is y=f(x), what equation describes B’s graph? Describe how you determined this equation.

    • In general, if the graph of y=f(x) is translated horizontally h units and vertically k units, what is the equation of this translated function?


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Example A Statistics Page. Place Time A, Time B, Distance A, and Distance B each in a list.

  • Describe how the graph of f(x)=4+2(x-3) is a translation of the graph of f(x)=2x.

  • The graph of f(x)=4+2(x-3) passes through the point (3, 4). Consider this point to be the translated image of (0, 0) on f(x)=2x.

  • The point is translated right 3 units and up 4 units from its original location, so the graph of f(x)=4+2(x-3) is the graph of f(x)=2x translated right 3 units and up 4 units.


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  • Note that you can distribute and combine like terms in Statistics Page. Place Time A, Time B, Distance A, and Distance B each in a list. f(x)=4+2(x-3) to get f(x)=-2+2x.

  • The fact that these two equations are equivalent means that translating the graph of f(x)=2x right 3 units and up 4 units is equivalent to translating the line down 2 units.

  • In the graph in the example, this appears to be true.


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Example B Statistics Page. Place Time A, Time B, Distance A, and Distance B each in a list.

  • The red graph is a translation of the graph of function f.

  • Write an equation for the red function in terms of f(x).

    • Any point on f(x) can be matched with a point right 2 units and down 3 units on the red function.

    • For example, the image of (1,2) is (1,1).

    • One notation to show this translation is (x, y) → (x+2, y-3).

    • The equation of the red graph can be written y-(-3)=f(x-2), or y+3=f(x-2) or y=-3+f(x-2)


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