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 Oct14 answered Integration by parts: $\int xe^{-x}dx$ Oct14 comment How do we explain to students that division by a vector does not make sense? It certainly depends on what your multiplication is. Oct11 answered Geometric significance of $\sqrt{A^2 + B^2}$ in general equation of line, if any? Oct11 revised Geometric significance of $\sqrt{A^2 + B^2}$ in general equation of line, if any? typo Oct6 comment How to tell if multivariable function is odd? You just do. It's something you get used to after a while. After all, it's usually pretty easy to tell at a glance whether a function is odd. Oct5 comment Let $f,g$ be two distinct functions from $[0,1]$ to $(0, +\infty)$ such that $\int_{0}^{1} g = \int_{0}^{1} f$. Are those powers or derivatives? Oct1 revised Is the function $f(x)= {\sin x \over x}$ uniformly continuous over $\mathbb{R}$? added 11 characters in body Oct1 awarded Benefactor Oct1 accepted Why do we think of a vector as being the same as a differential operator? Sep30 comment Why do we think of a vector as being the same as a differential operator? I think I like your answer the most, but I'll wait a bit longer just in case another one pops up. Thanks! Sep29 comment Why do we think of a vector as being the same as a differential operator? Why is it the only sensible definition? If I have a point $p \in M$ and an open set $U \ni p$ with local coordinates $\phi: U \to \mathbb{R}^n$, then I can choose (for example) the standard basis of $\mathbb{R}^n$, and if I want to use another coordinate system, the vectors transform as dictated by the Jacobian, so my basis is well-defined regardless of the coordinates. Isn't this right? Sep28 comment is it true that $\det(I+A)>0$ , if $\det(A)>0$? +1 This is the simplest example, I think. Sep26 comment How to show that the is a $1-1$ correspondence between real numbers and the set of points of a line in the Euclidean plane? I think the question is about the relationship between $\mathbb{R}$ as a set of numbers and the geometric notion of a line, but I could be wrong. Sep26 revised Surprising identities / equations added 29 characters in body Sep26 awarded Promoter Sep25 comment Why do we think of a vector as being the same as a differential operator? @GeorgesElencwajg: If you think that's the answer, then post it as so. But I still wonder: if there's no reason to make no distinction, why does the author make no distinction? Sep23 asked Why do we think of a vector as being the same as a differential operator? Sep22 comment Simplify the expression. Isn't this needlessly complicated? Why not just substract exponents at the second equality, if you're going to do it later anyway? Sep21 comment Integrating $\sec^2 x$ from first principles Why would you not know that? It's not magic, it's very easy to derive. Sep21 comment lim calculus problem with infinity @vilbur: $\frac{n+5-2}{n+5} = \frac{(n+5)-2}{n+5} = \frac{n+5}{n+5} - \frac{2}{n+5} = 1-\frac2{n+5}$.