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Math Help - calc3 (projectile motion)

  1. #1
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    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??

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  2. #2
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    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
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