The equation of projectile is y = ax - bx2. its horizontal range is?

Y = a x - b x^2

Range is a/b

y = tan Ф x - g x² / 2 u² cos² Ф

tan Ф = a - equation 1

b = g / 2u² cos² Ф so u² cos² Ф = g / 2b - equation 2

R = u cos Ф * 2 * u sin Ф / g = 2/g sinФ u² cos Ф

= 2 / g tan Ф u² cos² Ф by using equation 1 and equation 2

= (2 / g) a (g / 2b) = a / b