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Math Help - A little stuck

  1. #1
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    A little stuck

    I have a problem doing the Binomial Probability. Yes, I know the formula, P(x)=n!/(n-x)!x! yada yada yada.


    Now I am I watching the video, it is telling me that 12!/5!(7!), I should be getting 792, for my first step. And I inputed this in my TI84 I get 2.01180672e10..... Where did I go wrong? I inputed the factorial symbol but yet I got it wrong.
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  2. #2
    MHF Contributor MarkFL's Avatar
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    Re: A little stuck

    You need to input 12!/(5!*7!)...your calculator correctly interpreted your original input as (12!/5!)*7!

    You need to enclose the denominator within parentheses, and you will get 792 as the result.
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  3. #3
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    Re: A little stuck

    Your notation is simply incorrect.

    12!/5!(7!)

    Is actually equal to

    \frac{12!}{5!} \cdot 7! = that big number you mentioned

    When what you want is

    \frac{12!}{5! \cdot 7!} = 792

    Always remember that parentheses may be needed to make clear what you're trying to do.
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  4. #4
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    Re: A little stuck

    I C, thank you, the video did not have those parentheses.
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  5. #5
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    Re: A little stuck

    Okay, I am actually stuck on this problem.

    Using the Binomial Formula.

    N=3, X=2 P=0.60, there is no Q for this one.

    After plugging my info, this is what I got.

    3!/(2! x 2!) Times p^x, which is 0.60^2..

    For 3!/(2!x2!) I got 1.5... For 0.60^2 I got.36... Which I multiplied 1.5 with .36 I got .54... The answer is suppose to 0.432.. Just wondering where I went wrong. TY

    Yeah, I still see my error, but the calculation still came out wrong.
    Last edited by Past45; September 16th 2012 at 10:17 PM.
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  6. #6
    MHF Contributor MarkFL's Avatar
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    Re: A little stuck

    For Q you would use Q=1-P and your probability would be:

    P(X)={3 \choose 2}(0.6)^2(0.4)=0.432
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  7. #7
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    Re: A little stuck

    Thanks, the book didn't even mention that that it becomes Q=1-P.

    Thanks again.
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  8. #8
    MHF Contributor MarkFL's Avatar
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    Re: A little stuck

    This comes from the fact that it is certain that event X will either occur or it will not occur. If we let P be the probability that event X occurs and Q be probability that event X does not occur, then we may state:

    Q + P = 1 or Q = 1 - P.

    This is sometimes referred to as the complementation rule:

    P(E) = 1 - P(not E)
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  9. #9
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    Re: A little stuck

    Thanks again. For some reason, I am doing all the right steps on a different problem and my answer is wrong for this problem.

    N=8, X=5,P=.90

    After doing my plugging in and etc, I have 8!/(3!X5!) *(.90)^2(.1)... I did the right steps didn't I? The answer was suppose to be 0.033, which is clearly off from what I had(4.536).
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  10. #10
    MHF Contributor MarkFL's Avatar
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    Re: A little stuck

    The binomial probability formula is:

    P(x)={n \choose x}p^x(1-p)^{n-x}

    So, you want:

    P(5)={8 \choose 5}(0.9)^5(0.1)^3\approx0.033
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