Explanation

(c) Equation of projectile
$y = x - {\left( \frac{5}{9} \right)x}^{2}$
Standard equation
$y = x\tan{\theta - \frac{g}{{2u}^{2}\cos^{2}\ \theta} \bullet x^{2}}$
$On\ comparing,\ we\ get$
$\tan{\theta = 10}$
$and\ \frac{g}{{2u}^{2}\cos^{2}\theta} = \frac{5}{9}$
$or\ \ {10u}^{2}\cos^{2}\theta = 9g$
$g = 10\ {ms}^{- 2}$
$\therefore\ u^{2}\cos^{2}\theta = 9$
$range\ of\ projecticle\ \ R = \frac{{2u}^{2}\tan\theta \bullet \cos\theta}{g}$
$= \frac{{2u}^{2}\tan\theta \bullet \cos\theta}{g}$
$(\because\sin{\theta = \tan{\theta \bullet \cos{\theta)}}}$
$\frac{{2(u}^{2}\cos^{2}\theta) \bullet \tan\theta}{g}$
$= \frac{2 \times 9 \times 10}{10} = 18\ m$