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Before the midterm

A sign slip

The habit: Carry the sign through every line and check it against the sign the answer must have.

83 practice questions are built to catch it, in 17 topics.

What it looks like

A question from the definition of the derivative, with the answer this slip gives beside the right one.

Introductory2 marks · 3 min

A company’s weekly cost C(x)C(x) of making xx bicycle helmets is a linear function of xx. A week in which it makes 200 helmets costs $5400, and a week in which it makes 350 helmets costs $7650. Find the company’s fixed cost: the weekly cost when no helmets are made.

Give the answer in dollars.

With the slip
8400\displaystyle 8400
Worked through
C(x)=15x+2400;fixed cost $2400\displaystyle C(x) = 15x + 2400;\quad \text{fixed cost } \$2400

From 200 helmets down to 0 the cost falls: the 200 × $15 is taken off $5400, not added.

See the worked solution

Where it comes up

Practise against it

Each of these has an answer this slip gives. Work them on paper: if you fall into it, the verdict names it and the question comes back in review.

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