How will you measure the height of an accessible object.

Explanation

An object whose distance from the point of observation can not be measured is called inaccessible object e.g. a tree or building on the other side of a river .
Let AB be an inaccessible tower of height h. Let `alpha` and `beta` be the angles of elevation subtended at observation points C and D respectively. The distance `DC=d` can be measured directly with the help of metre rod or measuring tape
From `DeltaACB, cot alpha=(l)/(h)`.............(i)
From `DeltaADB cot beta =(l+d)/(h)` ............(ii)
Subtracting (i) from (ii)
`(cot beta-cot alpha)=(l+d)/(h)-(l)/(h)=(d)/(h)` or `h=(d)/((cotbeta-cotalpha))`
Now by knowing the values of `alpha, beta` the height h of inaccessible object can be calculated