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Sep
28
comment is it true that $\det(I+A)>0$ , if $\det(A)>0$?
+1 This is the simplest example, I think.
Sep
26
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.
Sep
25
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?
Sep
22
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?
Sep
21
comment Integrating $\sec^2 x$ from first principles
Why would you not know that? It's not magic, it's very easy to derive.
Sep
21
comment lim calculus problem with infinity
@vilbur: $\frac{n+5-2}{n+5} = \frac{(n+5)-2}{n+5} = \frac{n+5}{n+5} - \frac{2}{n+5} = 1-\frac2{n+5}$.
Sep
21
comment lim calculus problem with infinity
Also, the nth-root of n goes to $1$. If you don't believe me, take the logarithm of the expression.
Sep
21
comment lim calculus problem with infinity
The fraction goes to $1$ but the exponent goes to $\infty$; this doesn't go to $1$ but rather is an indeterminate case, as evidenced by the well known fact that $(1+\frac1{n})^n \to e$.
Sep
18
comment Does infinite time = time with no end = never?
The short answer is that if it takes an infinite amount of time to reach point B, then the object never reaches it. Of course, in the real world, no matter how precise your measurement is, there will be some time after which the object is closer to B than your instrument can distinguish.
Sep
10
comment Infinite amount of additions, finite sum?
@ZoltánSchmidt: While I found your question interesting, I think at some point you just need to accept that adding an infinite amount of terms can result in a finite result, and the intuition will come later. Someone on this site once quoted something along the lines of "In math, we don't understand things; we just get used to them". Try to work out the $\sum \frac1{2^n}$ example. It's easy to do and it will give you some insight as to why the total sum is $2$ and not $\infty$.
Sep
10
comment Is it true that $ \sum \limits_{i=1}^{\infty} f(i) = \lim_{n \to \infty} \sum \limits_{i=1}^{n} f(i) $?
By the way, it doesn't make sense to ask if this holds for all $n$, since $n$ isn't a free variable.
Sep
8
comment $f(x)=f(x^2+ 1/4)$ , $f$ is continuous from $\mathbb{R}$ to $\mathbb{R}$
$f$ can't be one-to-one, since for example $f(0) = f(\frac14)$.
Sep
7
comment Problem with differentiation as a concept.
Small correction: The definition is either $\lim_{h \to 0} \frac{f(x+h)-f(x)}{h}$ or $\lim_{x \to x_0} \frac{f(x)-f(x_0)}{x-x_0}$.
Sep
4
comment Integration of function
The binomial theorem proper only works when the power is a positive integer. You could get a series expansion, but otherwise I doubt this has a closed form. If it was a definite integral, you may be able to relate to the beta function using the substitution $u=\frac{a}{x}$.
Sep
4
comment Integration of function
Please make sure I did the LaTeX right. Is it $1+\frac{a}{x}$ or $\frac{1+a}{x}$?
Sep
2
comment Easy way to find the streamlines
If you're not supposed to do it rigorously, then sketching $\mathbf{F}$ at a few select points can give you an idea.
Sep
2
comment What's the Period of This Function?
Try graphing it, for particular values of $k$.
Aug
24
comment why $x^2 = y^3$ is not smooth?
Usually you require that the derivative is never zero, which in this case happens at $t = 0$.
Aug
23
comment How to indicate keeping the sign when squaring a number.
While $x \cdot |x|$ certainly works, there's nothing wrong with the cases definition, and it's actually easier to understand.
Aug
16
comment Can $0\le\lim_{t\rightarrow+\infty}\int_{0}^tf(\tau)\mathrm{d}\tau\le a$ imply $\lim_{t\rightarrow+\infty}\int_{0}^tf(\tau)\mathrm{d}\tau$ exists?
The issue here is that you ask if from $0 \le \lim \int_0^t f \le a$ we can deduce the limit in question ($\lim \int_0^t f$) exists. But if it turns out it doesn't exist, then the question didn't make sense in the first place. I'm not sure what the best and rigorous way to describe the same meaning is, because the way I see it, there is no meaning. I hope this doesn't sound rude; I'm just trying to use the mathematical terms. The question as stated has no meaning because you are using something that you don't know exists in a way that can only be used if it in fact exists.