If someone would be willing to write like a step by step procedure in dumb people terms for how to find them both I would be so grateful I'm so lost so far I get that you do the ratio or the root test and then you do something with the answer to that

  • ThatsNotPraxis [he/him]
    ·
    edit-2
    4 years ago

    A simple process for finding the radius of convergence:

    1.) Observe the point where the series is != 0 and set the sequence of the series to some "a_n" .

    2.) Apply the ratio/root test

    3.) Simplify as much as possible. The goal here being to simplify the sequence so you have some clear idea of how it behaves at the end points. I'm not really sure how to word this differently, but like if during the process of evaluating, lets say, a ratio test: lim n-> infinity |3(x-2)(n+1)/3(x-2)n| you can see that the 3's cancel, and (x-2)^n+1 / (x-2)^n simplifies to just |x-2| (since division of exponential is the same as subtracting their powers, you can review this in algebra)

    4.) Using our previous little example |x-2| < 1 converges and diverges when |x-2| > 1 (these are just the property of the ratio test) so the radius of convergence is R = 1.

    To answer the part about how do you know if it's a bounded and monotonic sequence:

    A bounded sequence is a sequence which has a definite lower bound and definite upper bound. And a monotonic sequence is one which is either non-decreasing or non-increasing so {1/n} is bounded and monotonic because {1/n} is bounded above by a_1 (a_1 = 1) and bounded below by 0 since the sequence never gets to 0 and thus never goes lower than 0. There is not a single element in the sequence which is greater than the previous one, the sequence is {1, 1/2, 1/3 ...} , so it is non-increasing and that makes it monotonic.