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 Jul 25 comment inverse trigonometric equation $\displaystyle \tan^{-1}{x}+\cot^{-1}{x}=\frac{\pi}{2}$ Hint: $\cot x = \tan(\frac{\pi}{2} - x)$. Jul 16 comment Are all existence proofs by contradiction? Jul 4 comment How to distinguish walking on a sphere or on a torus? @dotancohen: Even if they could look up, people living on the outer parts of Planet Doughnut wouldn't have a view of the hole. Apr 22 answered How many days are there in 70 years? Apr 20 comment Kid's homework: 4 equations 5 unknowns? Going crazy! @metacompactness: Not necessarily. For example, there could be half-siblings on this trip, with 1 father and 2 mothers. Or cases where one parent is unable to attend. Apr 5 answered how to find the root in $x^3-x^2+6x+24$? Apr 5 comment Derivatives of exponential functions There's no $x$ in your function. Did you mean dt instead of dx? Apr 5 answered Is $e^{-x}$ Lipschitz on the positive reals? Apr 5 comment How can a piece of A4 paper be folded in exactly three equal parts? It would be interesting to see an answer that relies on the $\sqrt{2}:1$ ratio of ISO paper sizes. Mar 29 comment Why is there a different button for 'minus' and 'negative' on a calculator? @Rahul: Based on your comment, I assume that you have an RPN calculator. However, I've also seen distinct - and ⁻ buttons on infix-notation calculators like the TI-8x series, on which there's no obvious need to distinguish them. Mar 29 comment Why is there a different button for 'minus' and 'negative' on a calculator? This doesn't really answer the question. Although unary and binary minus are indeed distinct operations, a lot of programming languages, spreadsheet programs, etc., do manage to use the same symbol - for both of them. Mar 29 awarded Popular Question Mar 29 comment Can a piece of A4 paper be folded so that it's thick enough to reach the moon? Assuming a typical 0.1 mm thickness, how large would a sheet of paper need to be so that it could be folded 42 times? Mar 28 comment Can a piece of A4 paper be folded so that it's thick enough to reach the moon? Just to verify, I measured a package of 500 sheets of paper, and it was about 5 cm thick. So, 0.1 mm per sheet is a good estimate. Mar 28 comment Can a piece of A4 paper be folded so that it's thick enough to reach the moon? A piece of A4 paper cut in half 42 consecutive times would be A46. Mar 28 comment Why is the $O$ (zero) matrix important? Do you understand why the number 0 is important? Mar 28 answered Why do we limit the definition of a function? Mar 21 awarded Great Answer Mar 21 awarded Citizen Patrol Mar 21 answered Is $\frac{0}{0}$ different from $\frac{1}{0}$?