Recent content by Elucidus

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    Why is it difficult to integrate x^x

    I think you're in for a lot of swearing and bloody knuckles. Since x^x is positive for all x > 0 then x^{(x^x)} is continuous for x > 0 and therefore Riemann integrable. But trying to find any sort of friendly resolution to it is a fool's errand. Numeric approximation is your best hope since...
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    Set Theory: Prove the set of complex numbers is uncountable

    Cantor proved that the real algebraiic numbers are denumerable in his 1874 paper "On a Property of the Totality of All Real Algebraic Numbers" the proof of which is nicely summarized in "The Calculus Gallery" by William Dunham. The Cartesian product of denumerable sets is also denumerable by...
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    Proving Continuity: Find Function Discontinuous at 0,1/2,1/3...

    The function f(x) = \left\{ \begin{array}{rl} 1, & x \text{ rational} \\ 0, & x \text{ irrational} \end{array} (aka the rational comb function) fails since it is not continuous anywhere. The function sought needs to be continuous everywhere except at 1, 1/2, 1/3, 1/4, etc. The...
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    Proving Continuity: Find Function Discontinuous at 0,1/2,1/3...

    I will point out the functions that are being cooked up are discontinuous at the natural numbers, but the original challenge was for the function to be discontinuous at x = 1, 1/2, 1/3, 1/4,... , but continuous at x = 0 and at all other numbers (I assume for all other real numbers). I have...
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    Discrete math:propositional logic

    Your tables are not clear enough for me to make out exactly what you are doing. I'm also not convinved there is a precedence rule between \Rightarrow and \Leftrightarrow. Part (c) may end up being ambiguous. --Elucidus
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    Solving Summations with Modified Exponents

    Assuming |r| < 1 then \sum_{n=0}^{\infty} r^n = \frac{1}{1-r} Differentiation both sides with respect to r gives: \sum_{n=1}^{\infty} n \cdot r^{n-1} = \frac{1}{(1-r)^2} This should give you a push in the right direction. (Warning: Be careful of your initial index.) --Elucidus
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    Darivative problems which one is wrong?

    I believe HallsofIvy means question 3 here. --Elucidus
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    I have a question about the value of limit.

    When an expression evaluates to something of the form 0/0, it is called an indeterminate quotient. There are more advanced methods (usually seen midway through Calculus I) that can handle these better. It is unfortunately the case that nothing about the limit can be deduced from the knowledge...
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    Solve for x: Log x + (Log x)^2=0

    x = 16 is a solution of \log_4(x^2) = (\log_4 x)^2. Both sides equal 4. HallsofIvy's comment though gives the hint for turning this into a (factorable) quadratic equation in log4x which has 2 solutions, namely 1 and 16. --Elucidus
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    How Many Possible Committees Can Be Chosen from a Group of 8 Men and 9 Women?

    Hints: (a) This is a combination, (8+9) choose 7. (b) (committee of 7 with 6 women) + (committee of 7 with 7 women) (c) (all committees) - (those with both Bob and Alice). --Elucidus
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    Darivative problems which one is wrong?

    Two of your answers are incorrect. Question 4 is correct, but question 6 is not. Use the hint in number 6 and let x = 0. Do you see why it must be false? I leave you to find the other incorrect answer. --Elucidus
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    Please tell me how they got this derivatives

    This is false. -ab means -(ab). If you meant raising -a to the bth power, you have to write (-a)b. --Elucidus
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    Proof of p→(q→r)→((p→q)→r)

    I assume you are asked to show p \rightarrow (q \rightarrow r) \Rightarrow (p \rightarrow q) \rightarrow r. Proofs involving conclusions of the form "if A then B" are usually best proven by assuming the premises of the claim and A and then showing B is a consequence. Basically: Given p...
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    Find F: R->R satisfying F(x+y)=F(x)+F(y) and F(xy)=F(x)F(y)

    Yeah. I knew I needed (5) but blew it on (4). Monotonicity was staring at me, taunting me. I went and looked back at my scratch work and right there on page 2 is u \leq v \Rightarrow f(u) \leq f(v) I will mention though that (1) and (2) should be 1) Prove that f(1) = 0 or 1. 2)...
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    Solution to Find Limit as x Approaches -5 from Left of 3x/2x+10

    You can use the following theorems: For any real number a and positive integer m: \lim_{x \rightarrow a^+} \frac{1}{(x-a)^m} = \infty \lim_{x \rightarrow a^-} \frac{1}{(x-a)^m} = \left\{ \begin{array}{rl} \infty, & \text{if }m \text{ is even} \\ -\infty, & \text{if }m \text{ is odd}...
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