1. calc3 (projectile motion)

what two angles of elevation will enable a projectile to reach a target 16km downrange on the same level as the gun if the projetile's initial speed is 400m/sec?

i thought i did it right , but my answers were both +/- 10 degree's different then the books answers.. using the formula i provided, can someone show me what i m doing wrong??

2. Don't change the $\sin 2\alpha \mbox{for} \sin\alpha \cos\alpha$ as there is no need to do this. If you don't substitute that in then you get the right answer:

$\frac{16000*9.8}{160000} = \sin 2\alpha$
$2\alpha = \sin ^{-1} (\frac{16000*9.8}{160000})$
$2\alpha = 78.52$
$\alpha = 39.3$

Then account for the other answer where:
$180 -2\alpha = 101.48$
$\alpha = 50.7$