5-5 Direct Variation

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5-5 Direct Variation
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Ex. 1 Is Direct Variation?

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Your Practice

• Is 3y 4x 8 an example of direct variation?
• No, does not equal zero.

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Ex. 2 Writing an Equation

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Your Practice

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Ex. 3 Story Problems

• Your distance from lightning varies directly with
  the time it takes you to hear thunder. If you
  hear thunder 10 seconds after you see lightning,
  you are about 2 miles from the lightning. Write
  an equation for the relationship between time and
  distance.
• Whatever comes before varies directly is y
• y 2
• Whatever comes after varies directly is x
• x 10
• Put into y kx to find k and write equation
• 2 k(10)
• 2 10k
• 10 10
• 1 k
• 5
• So y 1/5k

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Your practice

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Ex. 4 Direct Variation and Tables

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x y y/x decimal
-3 2.25 2.25/3 -0.75
1 -0.75 -.75/1 -0.75
4 -3 -3/4 -0.75
6 -4.5 -4.5/6 -0.75
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Your Practice

• Is the table show direct variation and if so,
  what is the equation?
• Check y/x and decimal to be the same
• k 1.8
• y 1.8x

x y
3 5.4
7 12.6
12 21.6
y/x decimal
5.4/3 1.8
12.6/7 1.8
21.6/12 1.8