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If you remember the following formula : H / R = tan Ф / 4 , then its quick. H = R => Ф = tan⁻¹ 4. =============== Derivation:

In two dimensional projection of a particle in Earth's gravitational field, we have the following equations :

x = u Cos Ф * t --- (3) y = u Sin Ф * t - 1/2 g t² --- (1) v_x = velocity in horizontal direction : u * cosФ v_y = vertical velocity upwards = u SinФ - g t v_y = 0 at t = u SinФ/ g

Then by (1) above, y at t = u SinФ/ g is equal to = H H = u² Sin²Ф /2g ---(2)

In the time duration t = u SinФ/g, the projectile travels, a distance of half the Range. Using (3) we get : R/2 = u² CosФ SinФ / g R = u² Sin2Ф / g -- (4)

From (2) and (4) we get : H / R = Tan Ф / 4 ---- (5)

If H = R are equal, then the angle of projection = tan⁻¹ 4 = 75.96 deg.

====== sin A = 4 cos A Sin² A = 16 Cos² A cos² A = 1/17 Cos A = 1/√17 Sin A = 4/√17