$解：在? Rt △A HO ?中，?s in α =\frac {OH}{OA} ?$
$∴?OA=\frac {OH}{sin α}?$
$在? Rt △BHO ?中，?s in β=\frac {OH}{OB}?$
$∴?OB=\frac {OH}{sinβ}?$
$∵?AB=4?$
$∴?OA+OB=4，?即? \frac {OH}{sin α}+ \frac {OH}{sinβ}=4?$
$∴?OH =\frac {4sinαsinβ}{sinα+sinβ}(\mathrm {m})?$
