projectile motion

Bon!

You are operating a canon that can fire at approaching tanks. The canon can fire a projectile at any angle between 0 and 90 degrees. You wish to hit a tank that is changing position at speed u. It is coning toward you. At what angle should you aim if the tank is horizontal distance D away.

Given that

D= 1000 m
u= 10 m/s.
speed of projection 200m/s.


我係咁計的..

Let the horizontal displacement of the canon ball be x.
Let the angle of projection be y.
Let the time of flight be t.

200cosy * t = x ..........1
0 = 200siny * t - 5t^2 ......... 2
10t = 1000-x .......... 3

From 2 ,
t = 40siny ......... 4

Sub 4 into 1 and 3,
200cosy * 40siny = x .........5
10*40siny = 1000-x .........6

From 5 and 6,

200cosy * 40siny + 10 * 40siny = 1000
40siny(200cosy + 10) = 1000


跟住唔識solve落去 ..
求幫忙 ..
同埋有做錯既話請指正 , 謝