modeling using variation - unc charlottemath2.uncc.edu/~adow/1100/3.7.pdf · g = 20p replace k, the...
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Direct Variation
• If a situation is described by an equation in the form
• y = kx• where k is a constant, we say that y varies
directly as x. The number k is called the constant of variation.
Solving Variation Problems
1. Write an equation that describes the given English statement.
2. Substitute the given pair of values into the equation in step 1 and find the value of k.
3. Substitute the value of k into the equation in step 1.
4. Use the equation from step 3 to answer the problem's question.
Direct Variation with Powers
• y varies directly as the nth power of x if there exists some nonzero constant k such that
• y = kx n.• We also say that y is directly proportional
to the nth power of x.
Text ExampleThe amount of garbage, G, varies directly as the population, P. Allegheny County, Pennsylvania, has a population of 1.3 million and creates 26 million pounds of garbage each week. Find the weekly garbage produced by New York City with a population of 7.3 million.
SolutionStep 1 Write an equation. We know that y varies directly as x is expressed as
y = kx. By changing letters, we can write an equation that describes the following English statement: Garbage production, G, varies directly as the population, P.
G = kP
The amount of garbage, G, varies directly as the population, P. Allegheny County, Pennsylvania, has a population of 1.3 million and creates 26 million pounds of garbage each week. Find the weekly garbage produced by New York City with a population of 7.3 million. SolutionStep 2 Use the given values to find k. Allegheny County has a population of 1.3 million and creates 26 million pounds of garbage weekly. Substitute 26 for G and 1.3 for P in the direct variation equation. Then solve for k.26 = 1.3k26/1.3 = k 20 = k
Text Example cont.
The amount of garbage, G, varies directly as the population, P. Allegheny County, Pennsylvania, has a population of 1.3 million and creates 26 million pounds of garbage each week. Find the weekly garbage produced by New York City with a population of 7.3 million. SolutionStep 3 Substitute the value of k into the equation.
G = kP Use the equation from step 1.G = 20P Replace k, the constant of variation, with 20.
Text Example cont.
The amount of garbage, G, varies directly as the population, P. Allegheny County, Pennsylvania, has a population of 1.3 million and creates 26 million pounds of garbage each week. Find the weekly garbage produced by New York City with a population of 7.3 million.
SolutionStep 4 Answer the problem's question. New York City has a population of 7.3 million. To find its weekly garbage production, substitute 7.3 for P in G = 20P and solve for G.
G = 20P Use the equation from step 3.G = 20(7.3) Substitute 7.3 for P.G = 146
The weekly garbage produced by New York City weighs approximately 146 million pounds.
Text Example cont.
Inverse Variation
• If a situation is described by an equation in the form
• y = k/x
• where k is a constant, we say that y varies inversely as x. The number k is called the constant of variation.
Example
• Describe in words the variation shown by the given equation:
Solution:H varies directly as T and inversely as Q
Joint Variation
• Joint variation is a variation in which a variable varies directly as the product of two or more other variables. Thus, the equation y = kxz is read "y varies jointly as x and z."
ExampleAn object’s weight on the moon, M, varies directly as its
weight on Earth, E. A person who weighs 75 kilograms on Earth weighs 12 kilograms on the moon. What is the moon weight of a person who weighs 80 kilograms on Earth?
SolutionM = kE12 = k*75k = .16M = .16EM = .16 * 80M = 12.8 kilograms