Answer
$1.29km $
Work Step by Step
As shown in the figure, $\bar {AB}=2km, A=40^\circ, B=70^\circ $
Assume the height of the balloon is $ x $:
$\frac{x}{tan(40)}+\frac{x}{tan(70)}=2$,
Solve for x:
$ x=1.29km $
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