The angle of elevation changes by 0.0032 rad / sec
Let x (t) be the horizontal distance from you to bird.
Let θ be the angle of elevation of your head.
Then .tan θ = 20 /x
Differentiating
Sec² = dθ / dt = -20/x².dx/dt
dθ / dt = -20/x².dx/dt.cos²θ
dx/dt = -2m/s x = 110m
cos²θ = 110²/ (110²) + (20²) = 12100/12500
dθ / dt = 20/ (110)² x (-2) . 12100/12500
= 20 x 2/12500 = 0.0032 radian/s.
Hence ,
The angle of elevation changes by 0.0032 rad / sec
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