Mathematics
Grade 9
15 min
Write direct variation equations
Write direct variation equations
Tutorial Preview
1
Introduction & Learning Objectives
Learning Objectives
Define direct variation and identify the constant of variation.
Determine if a relationship between two variables represents a direct variation.
Write a direct variation equation when given two corresponding values.
Write a direct variation equation from a table of values.
Write a direct variation equation from a real-world scenario.
Use a direct variation equation to find an unknown value.
Ever notice how the more songs you download, the more storage space you use? 📱 That's a direct variation in action!
In this tutorial, you will learn how to recognize a special relationship called direct variation. We will focus on how to find the 'constant of variation' and use it to write a powerful equation that describes this relationship, allowing...
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Key Concepts & Vocabulary
TermDefinitionExample
Direct VariationA relationship between two variables, typically x and y, where as one variable increases, the other increases at a constant rate. Their ratio is always constant.If you earn $15 per hour, your total earnings (y) vary directly with the number of hours you work (x). The more hours you work, the more you earn.
Constant of VariationThe non-zero constant ratio in a direct variation, represented by the variable 'k'. It's the 'rate' that connects the two variables.In the $15 per hour pay scenario, the constant of variation (k) is 15, because the ratio of earnings to hours (y/x) is always 15.
Dependent VariableThe variable (usually y) whose value depends on the value of another variable.In `y = 3x`, the value of `y` depends on the valu...
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Core Formulas
The Direct Variation Equation
y = kx
This is the fundamental equation for any direct variation. 'y' and 'x' are the variables, and 'k' is the constant of variation. This equation shows that y is always a constant multiple of x.
The Formula for the Constant of Variation
k = y/x
To find the constant of variation (k), divide any corresponding y-value by its x-value. This formula is derived by rearranging the main equation `y = kx`.
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Challenging
If (a, b) and (c, d) are two ordered pairs of a direct variation relationship, where a, c ≠0, which of the following statements must be true?
A.a + c = b + d
B.b/a = d/c
C.ac = bd
D.a/b = d/c
Challenging
The circumference (C) of a circle varies directly with its diameter (d). A circle with a diameter of 10 cm has a circumference of approximately 31.4 cm. Write an equation for this relationship and use it to find the approximate diameter of a circle with a circumference of 78.5 cm.
A.= 10d; 7.85 cm
B.d = 3.14C; 246.49 cm
C.= 3.14d; 25 cm
D.= 0.318d; 246.8 cm
Challenging
The following table is supposed to represent a direct variation, but one y-value is incorrect. By identifying the constant of variation from the correct pairs, determine the correct direct variation equation.
x | 3 | 5 | 8 | 12
y | 10.5 | 17.5 | 29 | 42
A.y = 3.5x
B.y = 3.625x
C.y = 30x
D.y = x + 7.5
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