Which of the following pairs $(x, y)$ are NOT in direct proportion?
Which of the following pairs $(x, y)$ are NOT in direct proportion?
$(2, 6), (4, 12), (5, 15)$
$(1, 5), (2, 10), (3, 15)$
$(3, 9), (4, 12), (6, 20)$
$(1, 7), (2, 14), (3, 21)$
Solution
Check $\dfrac{x}{y}$. In (c): $\dfrac{3}{9} = \dfrac{1}{3}$, $\dfrac{4}{12} = \dfrac{1}{3}$, but $\dfrac{6}{20} = \dfrac{3}{10}$. Not constant, so not direct.