\ \

Sightings to the same point at water level directly under the bridge are taken from each side

of the long bridge,as indicated in the figure. \ \

\ \

The triangle angles are and .

One of the triangle side is .

The third angle is .

\

Use the law of sines rule: 

Consider .

Substitute , and in above rule,

Therefore the height can be calculated by \ \

The height of the bridge is . \ \