Intermediate Value Theorem

Key Questions

  • To answer this question, we need to know what the intermediate value theorem says.

    The theorem basically sates that:
    For a given continuous function f(x)f(x) in a given interval [a,b][a,b], for some yy between f(a)f(a) and f(b)f(b), there is a value cc in the interval to which f(c) = yf(c)=y.

    It's application to determining whether there is a solution in an interval is to test it's upper and lower bound.

    Let's say that our f(x)f(x) is such that f(x) = x^2 - 6*x + 8f(x)=x26x+8 and we want to know if there is a solution between 11 and 33 (in the [1,3][1,3] interval).
    f(1) = 3f(1)=3
    f(3) = -1f(3)=1
    From the theorem (since all polynomials are continuous), we know that there is a cc in [1,3][1,3] such that f(c) = 0f(c)=0 (-1 <= 0 <= 3103)//

    Hope it helps.

  • Answer:

    There are several definitions of continuous function, so I give you several...

    Explanation:

    Very roughly speaking, a continuous function is one whose graph can be drawn without lifting your pen from the paper. It has no discontinuities (jumps).

    Much more formally:

    If A sube RR then f(x):A->RR is continuous iff

    AA x in A, delta in RR, delta > 0, EE epsilon in RR, epsilon > 0 :

    AA x_1 in (x - epsilon, x + epsilon) nn A, f(x_1) in (f(x) - delta, f(x) + delta)

    That's rather a mouthful, but basically means that f(x) does not suddenly jump in value.

    Here's another definition:

    If A and B are any sets with a definition of open subsets, then f:A->B is continuous iff the pre-image of any open subset of B is an open subset of A.

    That is if B_1 sube B is an open subset of B and A_1 = { a in A : f(a) in B_1 }, then A_1 is an open subset of A.

  • Answer:

    It means that a if a continuous function (on an interval A) takes 2 distincts values f(a) and f(b) (a,b in A of course), then it will take all the values between f(a) and f(b).

    Explanation:

    In order to remember or understand it better, please know that the math vocabulary uses a lot of images. For instance, you can perfectly imagine an increasing function! It's the same here, with intermediate you can imagine something between 2 other things if you know what I mean. Don't hesitate to ask any questions if it's not clear!

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