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The formula is

$F = 9/5C + 32$

$F$ increases as $C$ increases and $F$ decreases as $C$ decreases, but the relation is not a direct proportionality.

In mathematics, a direct proportion has the form

$y = mx$,

where $m$ is a constant.

If the formula had been

$F = 9/5C$

it would have been a direct proportion.

However, the addition of the $+32$ term makes the formula no longer a direct proportion.

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