Question

from a point on a bridge across a river the angle of depressions of banks an opp side of river are 30 and 45respectively if the bridge is at height of 3m from banks find width of river?

Answer 1

if you  take  random point and across would be like this
             0( imagine "0" as a dot/point  as it more visible
              (0
                               0)
this would be the across of the point 
and the angle of depression is a downward slope and it says 30 degrees
 so join those two points and mark the internal angle as 30 degrees now the first point as 4 degrees .
now there are saying that the bridge is of a height 4m so draw a straight line from point B(second point).now join all the points.(note: it forms a triangle im unable to draw over here think carefully and observe properly).if u dont know this property now you would be learning one.the sum of interior angles of a triangle is 180 degrees.
so it 180-30-45=105 degrees.and the width is = area of a triangle =1/2 multiplied by base  multiplied by width(bredth) 
180= 1/2 multiplied by 3 mutiplied bywidth(note: in this context 180 would be the area as the length and other figures are unknown).
so its 360 divided by 3
=120m which is the width
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