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A standing charge of £9 is added to a bill whatever the usage, so the amount stays at £9 as usage rises. What is the rate of change of that line?
AIt is 9, matching the charge
BIt cannot be found from a charge
CIt is 0, since the amount never moves
DIt is 1, since one charge applies
Answer & Solution
Correct answer: C. It is 0, since the amount never moves
1. The output is £9 for every level of usage, so it never changes.
2. The change in output between any two points is therefore zero.
3. Zero divided by any change in input is zero, so the rate of change is 0.
4. Option A confuses the height of the flat line with its steepness, which are different things.
_Source: OpenStax College Algebra (CC BY 4.0), Ch 4 "Linear Functions", section 4.1 Linear Functions_
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