Javier
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 Oct 16 comment Fourier series for $\sin x$ is zero? But for $n=1$ you need to integrate $\sin^2 x$. Oct 11 comment Solve $3\log_{10}(x-15) = \left(\frac{1}{4}\right)^x$ @abc: "closed form" solution means a formula using only elementary operations (addition, substraction, multiplication, division) plus a handful of functions (exponentiation, roots and logarithms, trigonometric). Sep 30 comment Proving statement about dimensions of vector spaces @Shahab: fixed it. Sep 30 revised Proving statement about dimensions of vector spaces blockquote messed up my latex Sep 30 asked Proving statement about dimensions of vector spaces Sep 30 comment arclength of vector r You should mention that you can drop the absolute value because $t$ has to be positive. Sep 29 accepted Help with a linear algebra problem; defining a function with some properties Sep 29 comment Help with a linear algebra problem; defining a function with some properties Well, I feel stupid now. It's $(-2,1,1)$. Everything works out now. Thanks! Sep 29 comment Help with a linear algebra problem; defining a function with some properties That's what you get if you solve the equations defining $f^{-1}(1,1,1)$. I don't understand your edit; $(-2,1,0)$ itself doesn't belong to $f^{-1}(1,1,1)$. You have to add $(-1,2,0)$ to it. Sep 29 comment Help with a linear algebra problem; defining a function with some properties I got there, but when trying to solve the system $f(x) = (1,1,1)$, I get something different than what it should be. Sep 29 asked Help with a linear algebra problem; defining a function with some properties Sep 28 comment How to solve $\cos(\pi/2+t)\ge 0$? @Michael: your book has the same answer, it's just written complicated. Think about it; $t \in [\pi, 3\pi/2]$ or $t \in [3\pi/2, 2\pi]$ is the same thing as $t \in [\pi, 2\pi]$. Sep 28 comment How do I integrate $\sqrt{x^2+81}$ using trig substitution? Hyperbolic trig functions will work better in this case, probably. Sep 27 answered Multivariable Partial Derivative Sep 27 comment Multivariable Partial Derivative Why can't you just calculate it, instead of trying to check all those options? Sep 25 comment Solving simple modulus with a variable but use inegality operator I don't know if you meant $1$ or $-1$, but in any case $|4x-2| \ge 1$ is equivalent to $4x-2 \le -1$ or $4x-2 \ge 1$. Sep 25 comment Volume of a pyramid as a determinant? @HenningMakholm: Well, it's probably a tetrahedron. Sep 24 asked What is it called when a function is not continuous but still can have a derivative? Sep 24 awarded Citizen Patrol Sep 23 comment What is the relevance of the supremum in this question? @dukenukem: Pretty much. Sometimes the supremum is written a little differently, like this: $\displaystyle \sup_{x\in I}|f(x)-g(x)|$. But it's the same thing.