Step I : Horizontal range
R = ucos θ × t
where θ is the angle of inclination of gun to cover maximum range and t is the time. i.e., R = 150 cos θ × t …(i)
Step II : Vertical height,
H = usin θt + \(\frac{1}{2}gt^2\)
i.e., 100 = −150 sin θt − \(\frac{1}{2}\times10\times t^2\)
t2 – (30 sin θ)t – 20 = 0
By using x = \(\frac{-b\pm\sqrt{b^2-4ac}}{2a}\) for ax2 + bx + c
we get, t = \(\frac{30sin\,\theta\pm\sqrt{900sin^2\,\theta-1\times20\times1}}{1}\)
= 15sin θ ± \(\sqrt{225\,sin^2\theta-20}\) …(ii)
Step III : Putting the value of t from eqn. (ii) in eq, (i), we get,
R = 150 cos θ (15 sin θ + \(\sqrt{225\,sin^2\theta+20}\))
Putting various values of θ around 45º the value of θ comes to be 45.8º for maximum range.
(∵ θ = 45º for maximum range had the shot been fired from ground itself.)