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 Sep29 asked Help with a linear algebra problem; defining a function with some properties Sep28 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 $t \in [\pi, 2\pi]$. Sep28 comment How do I integrate $\sqrt{x^2+81}$ using trig substitution? Hyperbolic trig functions will work better in this case, probably. Sep27 answered Multivariable Partial Derivative Sep27 comment Multivariable Partial Derivative Why can't you just calculate it, instead of trying to check all those options? Sep25 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 $4x-2 \ge 1$. Sep25 comment Volume of a pyramid as a determinant? @HenningMakholm: Well, it's probably a tetrahedron. Sep24 asked What is it called when a function is not continuous but still can have a derivative? Sep24 awarded Citizen Patrol Sep23 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. Sep23 answered What is the relevance of the supremum in this question? Sep23 suggested rejected edit on How to go about solving $((1+iz)/(1-iz))^4 = 1/2 + i\sqrt3/2$? Sep23 comment How do you explain the appearance of a sine in the integral for calculating the surface area of a sphere? @Tpofofn: But where does the $2$, or, if you want, the $\sin \phi$ come from? Sep23 asked How do you explain the appearance of a sine in the integral for calculating the surface area of a sphere? Sep22 comment The zero vector If you keep the requirement that addition and scalar multiplication are closed and ask for the subspace to be non-empty, then as a consequence zero must be in it. We just include that axiom to get rid of the empty set. Sep21 comment Question about solving 2nd order linear differential equations What book are you using, by the way? Sep21 comment Question about solving 2nd order linear differential equations @Korgan: If it turns out you were right, please tell us, because this is the method I've seen in every differential equations book there is. I'd be very surprised if it was wrong. Sep20 revised Showing vectors span a vector space by definition Expanded answer Sep20 comment Showing vectors span a vector space by definition This indeed uses only the defintion. I'll edit to make it more explicit. Sep20 answered Showing vectors span a vector space by definition