An aeroplane leaves an airport and flies due north at a speed of $1000\  km$ per hour. At the same time, aeroplane leaves the same airport and flies due west at a speed of $1200\ km$ per hour. How far apart will be the two planes after $1\frac{1}{2}\text{hour}?$
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Let $C$ be the airport and let $A$ and $B$ be the two aerplanes flying north and west respectively Distance covered by $A$ in $1\frac{1}{2}=\frac{3}{2}\text{hours}$
$= AC$
$=1000\times\frac{3}{2}=1500\text{km}$
Distance covered by B in $1\frac{1}{2}=\frac{3}{2}\text{hours}$ hours
$= BC$
$=1200\times\frac{3}{2}=1800\text{km}$
In $\triangle\text{ABC},$
By Pythagoras theorem,
$AB^2 = AC^2 + BC^2$
$\Rightarrow AB^2= 1500^2 + 1800^2$
$\Rightarrow AB^2 = 2250000 + 3420000$
$\Rightarrow AB^2 = 5490000$
$\Rightarrow\text{AB}=300\sqrt{61}\text{km}$
Hence, after $1\frac{1}{2}$ hours, the planes $ A$ and $B$ are $300\sqrt{61}\text{km}$ apart.
art

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