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Proportional relationships and the constant k

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Learn it In a proportional relationship $y = kx$: the ratio $y \div x$ is the same for every pair, the graph is a straight line through (0, 0), and k is the constant of proportionality (the unit rate).

Example: x: 2, 4, 6 and y: 7, 14, 21.

$7 \div 2 = 3.5$, $14 \div 4 = 3.5$, $21 \div 6 = 3.5$.
Same every time, so it is proportional.
$k = 3.5$, so $y = 3.5x$.

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