Jointly Proportional
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Definition of Jointly Proportional
If a variable z is jointly proportional to a set of variable x and y, the z = kxy, where k is the constant of proportionality.
More About Jointly Proportional

Jointly proportional is also known as joint variation.
Example of Jointly Proportional

The kinetic energy, E, of a moving object is jointly proportional to its mass, m, and the square of its speed, v.i.e., E = k .m. v^{2}
Video Examples: Art of Problem Solving: Joint Proportion
Solved Example on Jointly Proportional
Ques: If x is jointly Proportional to y and z when x = 4, y = 8 and z = 9, then find the value of x when y = 2 and z = 18.

A. 1
B. 0
C. 2
D. 4
Correct Answer: C
Solution:

Step 1: x a k. y. z [joint proportional]
Step 2: x = k. y. z
Step 3: 4 = k. 8. 9 [Substituting the values for x, y, z.]
Step 4: k = 1/18[Simplify.]
Step 5: Therefore, x =1/18. 2. 18 [Substituting the values for k, y and z in x = k. y. z]
Step 6: So, x = 2 [Simplify.]
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