# Browsing category ask uncle colin

## Ask Uncle Colin: touching cubics

Dear Uncle Colin, I'm told that the graphs of the functions $f(x) = x^3 + (a+b)x^2 + 3x - 4$ and $g(x) = (x-3)^3 + 1$ touch, and I have to determine $a$ in terms of $b$. Where would I even start? - Touching A New Graph Except Numerically Troubling

## Ask Uncle Colin: An Infinite Sum

Dear Uncle Colin I've been asked to find $\sum_3^\infty \frac{1}{n^2-4}$. Obviously, I can split that into partial fractions, but then I get two series that diverge! What do I do? - Which Absolute Losers Like Infinite Series? Hi, WALLIS, and thanks for your message! Hey! I'm an absolute loser who

## Ask Uncle Colin: An oblique asymptote

Dear Uncle Colin I have been asked to describe how $y = \frac{3x^2-1}{3x+2}$ behaves as $x$ goes to infinity. My first answers, "$y$ goes to infinity" and "$y$ approaches $x$", were both wrong. Any ideas? - Both Options Reasonable, Erroneous Limits Hi, BOREL, and thanks for your message! My first

Dear Uncle Colin, What is $\frac{1}{\infty}$? - Calculating A Number, Though Outside Reals Hi, CANTOR, and thanks for your message! The short answer: it's undefined. The longer answer: Infinity is not a number. It's not something you're allowed to divide by. The calculation doesn't make sense, and writing it down

## Ask Uncle Colin: Reversing Fibonacci

Dear Uncle Colin, I'm aware of Binet's formula for finding the $n$th Fibonacci number, $F_n = \frac{\phi^n - (-\phi)^{-n}}{\sqrt{5}}$, and wondered if there was an inverse version - to find $n$ given a Fibonacci number. -- Fibonacci Explicit Inverse, Getting Extremely Nervous But Am Understanding More Hi, FEIGENBAUM, and thank

## Ask Uncle Colin: An Uncommon Logarithm

Dear Uncle Colin, I'm a bit stumped by a logs question with a variable base: $\log_{\sqrt[3]{x+3}}(x^3 + 10x^2 + 31x + 30) = 9$. I know the basics of logarithms, but this is currently beyond me. -- Obtaining Underwhelming Grade, Having To Review Every Definition Hello, OUGHTRED, and thanks for

Dear Uncle Colin, In a recent exam, I was invited to solve $12x^2 - 59x + 72=0$ without a calculator. Is that a reasonable thing to ask? Very Irate EdExcel-Taught Examinee Hi, VIETE, and I don't blame you for being cross - in a non-calculator exam, I'm not sure that

## Ask Uncle Colin: Spotting factors

Dear Uncle Colin, In a recent test, I stumbled across $9x^4 + \frac{1}{144x^4} + \frac{1}{2}$, which apparently factorises as $\left(3x^2 + \frac{1}{12x^2}\right)^2$. How on earth am I supposed to spot that?! - Feeling Almost Cheated, That's Only Reasonable Hi, FACTOR, and thanks for your message! I wouldn't instinctively spot that

## Ask Uncle Colin: An Enormous Sum

Dear Uncle Colin, I've got a question that asks me to find the coefficient of $x^5$ in $(1+x)^5 + (1+x)^6 + (1+x)^7 + ... + (1+x)^{100}$. I can easily work out the coefficient in each term (it's just $\nCr{k}{5}$), but I can't see an easy way to add them up.

Dear Uncle Colin, I'm pretty good with quadratic inequalities and pretty good with absolute values, but when I get the two together, I get confused. For example, I struggled with the set of values satisfying $x^2 -\left| 5x-3\right| < 2 + x$. Can you help? - Nasty Absolute Value Inequalities