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1h
comment Calculate the number of spheres
Hint: Have you done a search to find the density of the packing? Then compute the volume of each sphere and divide that into the occupied volume.
1d
comment What is the limit of the sum of “last half” part of harmonic series?
@Connor: yes, I dropped a sign. A $\LaTeX$ hint: if you put a backslash before ln, you get the right font, so \ln instead of ln gives $\ln$ instead of $ln$
1d
revised What is the limit of the sum of “last half” part of harmonic series?
correct
1d
answered What is the limit of the sum of “last half” part of harmonic series?
2d
comment Spaces $X$ in which every subset is either open or closed, and only $\varnothing$ and $X$ are clopen
Presumably you want each subset to be open or closed but not both. In the discrete topology, every subset is both open and closed.
2d
comment Transform square to rectangle with natural sizes.
If $A=2520=2^3\cdot 3^2\cdot 5\cdot7$ we know that $2^3$ and $3^2$ are very close and $3^2 \gt 2^3$, so you would do $2^3\cdot 7=56$ and $3^2\cdot 5=45$. But If you had $2^3\cdot 3^2\cdot1223\cdot 1229 \cdot 17393 \cdot 19103$ it isn't so obvious
2d
comment Python Read and write program
This seems to be another homework problem from the same user who posted math.stackexchange.com/questions/1412227/… It is again off topic and showing no effort. At least here the words are spelled correctly.
2d
comment Python Read A finle and returen capital letters program
This looks like a programming exercise,not math. It is also homework with no effort shown. -1 and vote to close- I wish I could give a third strike for the mistypes (I won't say typos-that is gentler) in the title. What have you tried and where is your problem?
2d
comment get rectangle size out of 2 corners and rotation
What do you mean by the size of the rectangle? Its dimensions, or its area? You need to go from one corner to the other by adding two vectors, one in the direction specified by the rotation and one perpendicular to that. Where does this come from? Sorry, but this seems like a homework problem dumbed down in language to avoid being called that.
Aug
27
awarded  puzzle
Aug
26
comment Magic of the number $2000+15$
What values is one allowed to use?
Aug
26
comment Intuition - For every % point that rates rise, a bond’s value will decline by its duration in years.
@BCLC: That is right. $(1+r)^t \approx 1+rt, (1+s)^{-t} \approx 1-st$
Aug
26
comment Intuition - For every % point that rates rise, a bond’s value will decline by its duration in years.
@BCLC: Not exactly. I am expanding each of the numerator and denominator separately, but the result is the same.
Aug
26
comment Equation to get the center point of the union of n ellipses?
The red area is the intersection, not union, of the ellipses
Aug
26
comment parallel resistors
You seem to be allowing parallel connections but not series ones, which seems curious. Can I use $1.2$ and $1.8$ to get $3$? This makes your problem trivial because I can just make a series of $1$s. Also it looks like you only allow one resistor of each value -is that true?
Aug
26
comment Find x in the triangle
What have you tried? I would let $AB=1, AD=a$, so $BC=1+a$ and note that $BD$ is part of both triangles. You should be able to find all the angles in terms of $x$. Now use the law of sines. Does this work? I haven't solved it, so no guarantees, but that is what I would try.
Aug
25
answered Math riddle - Forehead numbers
Aug
25
comment How many unique numbers can be obtained by adding two numbers from two different sequences?
You are right. I had read it incorrectly. So $ a_2+b_1=2a_1+D_1-k=a_1+b_2$ You can still have the $D$'s large and chosen so there are no coincidences in the sums (let all the $D_i$ be distinct powers of $2$ greater than $mk$, for example) so the number of distinct sums is maximal at $\frac 12m(m+1)$
Aug
25
answered Transform square to rectangle with natural sizes.
Aug
25
answered How many unique numbers can be obtained by adding two numbers from two different sequences?