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Exercise Solve. 5 = 3x x = 5 3

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Exercise Solve. 4 = k 1 3 k = 12

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Exercise Solve. y 25 3 5 = y = 15

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Exercise Solve. 3 5 25 x = x = 41 2 3

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Exercise If 4 = 6k, what is the value of 3k? 2

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Direct Variation A direct variation is formed by the variables x and y if the ratio y : x always equals a constant k, where k is a positive number.

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**Directly Proportional**

Variables are directly proportional when y is said to vary directly with x.

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**Constant of Proportionality**

The constant k is the constant of variation, or the constant of proportionality.

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x hours y miles y x 1 30 2 60 3 90 4 120

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Example 1 Does y vary directly with x in the following table? If so, find the constant of variation and write an equation for the direct variation. x y 1 3 3 9 5 15 7 21

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x y 1 3 3 9 5 15 7 21 = 3 1 y x 3 = 21 7 y x 3 = 9 3 y x 3 = y x 3 k y = kx = 15 5 y x 3 y = 3x

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Constant of Variation The constant of variation is the steady rate of change. The constant k is the constant of variation, or the constant of proportionality.

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Example 2 Indicate which equations represent a direct variation. If an equation describes a direct variation, give the constant of variation. f(x) = 2.2x direct variation; k = 2.2

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y = 4x − 1 This is not a direct variation; the variable must be a multiple of x. d = 45t This is a direct variation; k = 45. y = −2x This is not a direct variation; the coefficient of x must be positive.

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y = mx + b y = kx

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Example 3 Graph the direct variation y = 4x. x −1 1 y −4 4

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y x

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Example 4 Find k if y varies directly with x and y = 12 when x = . Write an equation for the direct variation. 1 2 y = kx k = 24 12 = k( ) 1 2 y = 24x 2(12) = k( )(2) 1 2

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Example 5 If y varies directly with x and y = 6 when x = 2, find y when x = . 2 3 y = kx y = 3x y = 3( ) 2 3 6 = k(2) 3 = k y = 2

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**Example Find k if y varies directly with x and y = 14 when x = 4.**

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**Example Find k if y varies directly with x and y = 15 when x = 2.**

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Example If y varies directly with x and y = 7 when x = 1, find y when x = 6. y = 42

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Example If y varies directly with x and y = 27 when x = 15, find y when x = 6. 54 5 y =

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Example Indicate which equations represent a direct variation. If an equation describes a direct variation, give the constant of variation. If not, explain.

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y = 4x Yes. k = 4. y = 3x + 5 No. The y-intercept is not zero. y = −4x No. The slope is not positive.

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Graphing Using Slope - Intercept STEPS : 1. Equation must be in y = mx + b form 2. Plot your y – intercept ( 0, y ) 3. Using your y – intercept as a starting.

Graphing Using Slope - Intercept STEPS : 1. Equation must be in y = mx + b form 2. Plot your y – intercept ( 0, y ) 3. Using your y – intercept as a starting.

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