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Direct Variation: Definition, Formula and Examples

Last Updated : 19 Jun, 2024
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Direct Variation is the relationship between two variables in which one is a constant multiple of the other and one changes according to the other. For example, if 'a' is directly varied to 'b' then 'a' and 'b' change accordingly.

In this article, we will learn about Direct Variation definition, Direct Variation formula, examples and others in detail.

What is Direct Variation?

Direct variation is a mathematical relationship between two variables in which one variable is a constant multiple of the other. In other words, when one variable changes, the other variable changes in proportion to it. This type of relationship can be described by the equation: y = kx

Direct Variation Example

Some examples of direct variation are:

  • Example 1: Speed of a car and the distance it travels are an example of direct variation. When the speed is increased, the distance travelled in a given amount of time increases as well. Similarly, as the car's speed lowers, the distance covered in that time interval reduces as well.
  • Example 2: Cost (C) of the fruits is directly proportional to the weight (w) of the fruits purchased.
  • Example 3: Total earnings (E) are directly proportional to the number of hours (h) worked.

Direct Variation Formula

Direct variation formula connects two numbers by establishing a mathematical relationship in which one variable is a constant multiple of the other. It is given as follows:

y = kx

where,

  • x and y are Quantities in Direct Proportion to Each Other
  • k is a Constant

Direct Variation Graph

In a direct variation relationship, the graph of the equation y=kxy=kx is a straight line that passes through the origin (0, 0). The direct variation graph is a linear graph and the image for the same is added below:

Constant-Proportion-2
Direct Variation Graph

Difference Between Direct Variation and Inverse Variation

Difference between direct variation and inverse variation is added in the table below:

FeatureDirect VariationInverse Variation
Definitiony is directly proportional to xy is inversely proportional to x
Equationy = kxy = k/x
Constantk is the constant of proportionalityk is the constant of proportionality
Graph TypeStraight line through the originHyperbola
RelationshipLinearNon-linear
Effect of Increasing xy increases if k > 0 and decreases if k < 0y decreases
Effect of Decreasing xy decreases if k > 0 and increases if k < 0y increases
ExampleDistance traveled at constant speed: d = vtSpeed and travel time: v = d/t
SlopeConstant slope kVariable slope, depends on x
AsymptotesNoneHorizontal and vertical axes
InterceptsPasses through Origin (0,0)No intercepts with the axes

Solving a Direct Variation

The formula y = kx is used to solve a direct variation. If the proportionality constant needs to be determined, divide y by x to get the answer. If k is known and either x or y must be found, these values can be replaced in the equation above to discover the unknown value.

Example: Find constant of proportionality if x = 69 and y = 23 have a direct variation.

Solution:

Formula for direct variation is y = kx or k = y/x

Hence, k = 69/23 = 1/3

Hence, constant of proportionality is 1/3

Read More:

Examples on Direct Variation

Example 1: Suppose y varies directly as x, and y = 72 when x = 8. Write a direct variation equation that relates x and y.

Solution:

We know that the direct variation formula is y = kx

Replace y with 72 and x with 8

72 = k(8)

k = 72/8

k = 9

Thus, direct variation equation is y = 9x

Example 2: Using the equation obtained in the above problem, find x when y = 63.

Solution:

As per the problem above, the direct variation equation is y = 9x

Replace y with 63

63 = 9x

x = 63/9

x = 7

Thus, x = 7, when y = 63

Example 3: The distance a jet travels vary directly as the number of hours it flies. Suppose it travelled 3420 miles in 6 hours. Write a direct variation equation for the distance d flown in time t.

Solution:

We know, Distance = Rate × Time

Let rate be r

As per the given info, we have

⇒ 3420 = r × 6

⇒ r = 3420/6

⇒ r = 570

Thus, direct variation equation is d = 570t

Example 4: In the above problem, estimate how many hours it will take for an airline to fly 6500 miles.

Solution:

As per above problem, d = 570t

Replace d with 6500

6500 = 570t

t = 6500/570

t ≈ 11.4

It would take the airline approximately 11.4 hours to fly 6500 miles

Example 5: Suppose y varies directly as x, and y = 98 when x = 14. Write a direct variation equation that relates x and y.

Solution:

We know that direct variation formula is y = kx

Replace y with 98 and x with 14

98 = k(14)

k = 98/14

k = 7

Thus, the direct variation equation is y = 7x

Example 6: In the above problem, find y when x = -4.

Solution:

From above problem, we have y = 7x

Replace x with -4 in the equation

y = 7(-4)

y = -28

Thus, y = -28 when x = -4

Example 7: If you post 5 messages on a message board, you receive 12 messages in return. Write a direct variation representing this info.

Solution:

We know that direct variation formula is y = kx

Replace y with 12 and x with 5

12 = k(5)

k = 12/5

Thus, direct variation equation is y = 12/5x

Practice Questions on Direct Variation

Q1. If y varies directly as x and y = 12 when x = 4, find the constant of variation and write the equation of the direct variation.

Q2. Amount of money earned (E) varies directly as the number of hours worked (h). If E = 150 dollars when h = 10 hours, what is the hourly wage? Write the equation that represents this relationship.

Q3. Cost (C) of buying apples varies directly as the weight (w) of the apples. If C = 8 dollars for 2 kilograms, how much would 5 kilograms of apples cost?

Q4. Volume (V) of a gas varies directly as its temperature (T). If the volume is 100 liters at 300 Kelvin, what is the volume at 450 Kelvin?

Q5. Distance (d) traveled by a car varies directly with the time (t) spent traveling. If the car travels 240 miles in 4 hours, how far will it travel in 7 hours?


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