Question
Find the mean proportion of: 5 and 80
✓
Answer
$
\begin{aligned}
& \text { Let } x \text { be the mean proportion of } 5 \text { and } 80, \\
& \text { then } 5: x:: x: 80 \\
& x^2=5 x 80 \\
& \Rightarrow x=\sqrt{5 \times 80} \\
& =\sqrt{400} \\
& =20 \\
& x=20
\end{aligned}
$Hence mean proportion $=20$.
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