Proportionality · TEKS 7.4A, 7.4C
Proportional relationships (y = kx)
Two quantities are proportional when y is always the same multiple of x. That multiple is k, the constant of proportionality.
Key steps
- k = y ÷ x. Check that it is the same in every row of the table.
- Equation: y = kx (no "+ b").
- Graph: a straight line through the origin (0, 0).
- Use k to find missing values: y = k × x.
Worked example
Table: 2 lb → $5, 4 lb → $10, 6 lb → $15. Write the equation.
- k = 5 ÷ 2 = 2.5 (10 ÷ 4 and 15 ÷ 6 also give 2.5).
- Answer: y = 2.5x
Watch out: The jump between rows is not k unless x goes up by 1 each time. Always divide y by x.
Practice
5 questions with new numbers every set. Get 4 of 5 to mark this topic as practiced.