A circle has radius \(5\) cm. Write down its circumference, giving your answer in terms of \(\pi\).
Question 1 Report
A circle has radius \(5\) cm.
Write down its circumference, giving your answer in terms of \(\pi\).
Answer Details
The circumference of a circle is \(C=2\pi r\), or equivalently \(\pi d\) using the diameter. With \(r=5\) cm,
\(C=2\times\pi\times 5=10\pi\) cm [B1] cao
Leaving the answer in terms of \(\pi\) means the symbol stays in the answer, so no decimal approximation is used and the value is exact.
The usual confusion is with the area formula \(A=\pi r^{2}\), which would give \(25\pi\). Circumference is a length, so it involves \(r\) to the first power and its unit is cm; area involves \(r^{2}\) and its unit would be cm\(^{2}\). Checking the unit of the answer is a quick way to confirm the right formula has been used.
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