trigonometry
posted by vad .
An aeroplane at an altitude of 200m observes the angles of depression of opposite points on the two banks of a river to be 45 degree and 60 degree. Find in metres, the width of river.

Let H be the altitude (which you already know), and D be the horizontal distance from the plane to the nearest bank of the river. Let w be the width of the river.
H/D = tan 60
H/(D + w) = tan 45
Not that the second equation implies that
H = D + w
and the first equation implies that
H = sqrt3 * D
Solve for w in terms of H, using substitution to eliminate D.
H = H/sqrt3 + w
H(1  1/sqrt3) = w 
Shut up

Right answer