Aim: What is an direct variation relationship? What is an inverse variation relationship?

Aim: Direct & Indirect Variations Course: Alg. 2 & Trig.

Aim: What is an direct variation relationship? What is an inverse variation relationship?Do Now: Fill in the missing values for the table below:

x 8 24 216 ?y 4 12 36 ? ?

?72108

400200

Describe the relationship between x and y.

x is twice the value of y

Write an algebraic equation that describes the relationship between x and y.

x = 2y or y =1/2x


Direct VariationIf a relationship exists between 2 variables

so that their ratio is constant the relationship is called a direct variation.

x seconds

y frames

12345

24487296120

Ex. As you watch a movie, 24 frames flash by every second.

Time (secs.)

# of

Fra

mes

40

80

100

120

1 2 3 4 5 60

y = 24x

y = kx

Constant of Variation

xy

k or

xy

24

linearequation

As x increases,y increases at aconstant rate


Direct Variation

p varies directly as t. If p = 42 when t = 7, find p when t = 4

Use a proportion to solve:

4742 p

7p = (42)(4)7p = 168p = 24

k = 6

Constant of Variation?

y = kx


xy

k or


Inverse VariationIf x and y vary inversely, then xy

= a nonzero constant, k.

xy = kxk = y


Ex. The number of days (x) needed tocomplete a job varies inversely as the number of workers (y) assigned to a job.If the job can be completed by 2 workersin 30 days.

What is the equation that represents thisrelationship?

What is the constant of variation? 60

xy = 60


Inverse VariationEx. The number of days (x) needed to complete a job varies inversely as the number of workers (y) assigned to a job.If the job can be completed by 2 workers in 30 days.

xy = 60

What other combinations of xy also satisfythis relationship?

x y2 303 204 155 126 10

How many days would it take 3 workers?

graph thisrelationship

xy = 60


Inverse Variation

Find x when y = 3, if y varies inversely as x and x = 4, when y = 16

(4)(16) = 64

Find the value of k

xy = kxk = y


x(3) = 64

64 121 33x


Graphing an Inverse Variation

xy = 60x

daysy

workers

2 303 204 155 126 10

The graph of an inverse variation relationship is a hyperbola whose center is the origin.

Note: as the days double (x 2) the numberof workers decreased by its reciprocal, 1/2.

30

20

10

10 20 300

workers

days


Graphing an Inverse Variation

x y-2 -30-3 -20-4 -15-5 -12-6 -10

xy = 60

xy = 60x y2 303 204 155 126 10

not valid for thisproblem


Model Problem

The cost of hiring a bus for a trip to NiagaraFalls is $400. The cost per person (x) variesinversely as the number of persons (y) whowill go on the trip.

a. find the cost per person if 25 go.b. find the persons who are going

if the cost per person is $12.50

xy = k k = $400

x(25) = 400

(cost per person) x (number of persons) = 400

x = 16

a. b. 12.50y = 400

y = 32


substitute to findconstant of I.V.

Model Problem

The intensity I of light received from a sourcevaries inversely as the square of the distance d from the source. If the light intensity is 4 foot-candles at 17 feet, find the light intensity at14 feet. Round your answer to the nearest100th. General equation of inverse variation

xy = k

(x - represents I) (y - represents the square of d) = kx • d2 = k

4 • 172 = k = 1156x • 142 = 1156

x = 5.90 foot candles


Model Problem

Draw the graph of xy = -12

x y-1 12-2 6-3 4-4 3-6 2

x y1 -122 -63 -44 -36 -2

y = xy = -x

lines of symmetry

y 12x

graphing calculator

combination of numbers that multiply and give -12


Regents Prep

If x varies inversely with y and x = -4 when y = 30, find x when y = 24.

1. -5 2. -3.2 3. 0.005 4. 180

If x varies directly as x and y = 20 when x = -4, find x when y = 50.

1. -250 2. -10 3. -0.8 4. 10


x varies directly as y. If x = 108 when y = 27, find y when x = 56

Use a proportion to solve:

y56

27108

108y = (56)(27) = 1512 y = 14

Based on the table at right, does y vary directly with x?

yx-314

1012

2.25-0.75

-3-7.5-9

kxy

75.0325.2

75.01

75.0

75.043

75.010

5.7

75.012

9

yes

y = -0.75x

y

Model Problem

Aim: What is an direct variation relationship? What is an inverse variation relationship?

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