About Direct Variation:

After learning about rational expressions, we generally discuss direct variation and inverse variation. Direct variation: y varies directly with x, if there is a constant k, such that y = kx. From this we know that if k > 0, then as x ↑ by 1 unit, y↑ by k units; similarly, as x ↓ by 1 unit, y ↓ by k units.


Test Objectives
  • Demonstrate the ability to find the value for the constant of variation (k)
  • Demonstrate the ability to solve a direct variation problem
  • Demonstrate the ability to solve a direct variation as a power problem
Direct Variation Practice Test:

#1:

Instructions: Solve each direct variation problem.

a) If y varies directly with x and y = 30 when x = 7, find y when x = 2/3


#2:

Instructions: Solve each direct variation problem.

a) If q varies directly with z and q = 9.45 when z = 3.5, find q when z = 7.3


#3:

Instructions: Solve each direct variation problem.

a) If y varies directly with x4 and y = 3888 when x = 6, find y when x = 3


#4:

Instructions: Solve each direct variation problem.

a) Hooke's law for an elastic spring tells us that the distance a spring stretches varies directly with the force applied: (D = kF). If a force of 150 pounds stretches a spring 32 inches, how much will a force of 225 pounds stretch the spring?


#5:

Instructions: Solve each direct variation problem.

a) The area of a circle varies directly with the square of its radius. A circle with a radius of 5 inches has an area of 78.53in2 (approx). What is the area of a circle with a radius of 4.7 inches?


Written Solutions:

#1:

Solutions:

a) y = 20/7


#2:

Solutions:

a) q = 19.71


#3:

Solutions:

a) y = 243


#4:

Solutions:

a) A force of 225 pounds would stretch the spring 48 inches.


#5:

Solutions:

a) A ≈ 69.36 in2