Corrine wrote temperatures in degrees Celsius and the equivalent temperatures in degrees Fahrenheit. Equivalent Temperatures Celsius negative 10 5 10 20 Fahrenheit 14 41 50 68 Which explains how Corrine could determine if the temperatures vary directly?

For direct variation, one quantity must equal a constant times the other, so the ratio of Fahrenheit to Celsius must be the same for every pair. Corrine can compare the equivalent-temperature ratios; if they are not equal, the temperatures do not vary directly. A consistent pattern or a straight-line graph alone is not enough, because a direct-variation graph must also pass through (0, 0).

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