# NQOTW

## Combinatorics: Multiple Methods, Subtle Wording

(A new question of the week) With few new questions of general interest available this week, I thought I’d go back a few months to a couple little questions on a topic we haven’t dealt with lately, combinatorics. We’ll have one question each on permutations and combinations, showing some subtlety in both the methods we …

## A Hole in a Cube

(A new question of the week) Here is an interesting little question about how drilling a hole affects volume and surface area. We’ll have one answer, and several explanations.

## Two Tricky Questions on Tangent Lines

(A new question of the week) Sometimes we have lots of quick questions and a number of long discussions, neither of which seems suitable for a post. This time I’ve chosen to combine two distantly related questions, one recent and one from several months ago, both involving tangent lines to functions.

## Two Sinusoidal Models

(A new question of the week) Two recent questions involved using trigonometric functions to model real-life (or nearly so) situations, one about breathing, the other about a Ferris wheel. Both can be done by writing a sinusoidal function; the second can be done in other interesting ways as well.

## Trig Identities: Where’d That Come From?

(A new question of the week) Proving a trigonometric identity can be a challenge; sometimes even when we read someone else’s proof, we can fail to see how they came up with a seemingly magical step. We’ll look at two such identities here, and consider how to bridge a gap when we are stuck.

## Function Transformations Revisited (II)

(A new question of the week) Last week we examined how a series of transformations affects the equation of a function, in order to write the equation from a graph, or vice versa. We touched on why it works the way it does, but this is something you need to look at from multiple perspectives …

## Function Transformations Revisited (I)

(A new question of the week) Transformations of functions, which we covered in January 2019 with a series of posts, is a frequent topic, which can be explained in a number of different ways. A recent discussion brought out some approaches that nicely supplement what we have said before. Here, the focus will be on …

## Solving a Triangle: What Went Wrong?

Trigonometry can be a powerful tool for solving sides and angles in triangles. But you have to be careful with it! We’ll look at a classic type of error in solving an SSA triangle, get three explanations, and then see how knowing the context of a question can change our answer – to the point …

## Cartesian Product of Sets

(A new question of the week) I had a long discussion recently about the Cartesian product of sets, answering questions like, “How is it Cartesian?” and “How is it a product?” I like discussions about the relationships between different concepts, and people who ask these little-but-big questions. We’ll be looking at about a quarter of …

## Why Does a² + b² = c² in a Hyperbola?

(A new question of the week) In an ellipse, $$\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$$ with focal distance c, parameters a, b, and c all make natural sense, and it is easy enough to see why $$a^2 = b^2 + c^2$$. But in the hyperbola, $$\frac{x^2}{a^2}-\frac{y^2}{b^2}=1$$, the equivalent relationship, $$a^2 + b^2 = c^2$$, is not nearly as natural, nor …