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Math Help - What is the probability density function?

  1. #1
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    What is the probability density function?

    f(x) = 5e^(ax) for x >= 0.

    Find a so that f(x) is a probability density function.

    My work:

    I can only get up to
    (5/a) lim [(e^at)-1] = 1
    t-> infinite

    Don't really know what to do after, but i think a has to be negative so that e^at can approach 0.
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  2. #2
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    Yes, a has to be negative. And if so, while t goes to +infinity, the limit of e^{at} is 0. Which should give you the solution for a...
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