Direct Variation

• y = mx (no b)

• Lines that go through the origin

• Y intercept of zero

• Slope also called constant of variation (variation constant)

Examples of Direct Variationy = -2x y = 2x

Checking for Direct Variation

Graph --> is it a straight line through the origin?

Equation --> does it fit y = mx?(may need to solve for y)

Do these lines illustrate direct variation?

y = -1/2x x = -3

What is the variation constant?y = -1/2x x = -3

Are these equations direct variation?

y = 5x

y = -3/4x

y = -2x - 7

Are these equations direct variation?

y = 5x --> Yes!

y = -3/4x --> Yes!

y = -2x - 7 --> No (y-int not 0)

Are these equations direct variation?

2x + y = 0

-x + y = -1

4x - 5y = 0

Are these equations direct variation?

2x + y = 0 --> y = -2x --> YES

-x + y = -1 --> y = x - 1 --> NO

4x - 5y = 0 --> y = 4/5x --> YES

Online Animation

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Homework

p256 #2-8 E,

11-22 C6,

58-62 E