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Nice examples of groups which are not obviously groups

Is mathematics one big tautology?

What is the difference between a point and a vector?

Divisibility by 7 rule, and Congruence Arithmetic Laws

What is the meaning of the third derivative of a function at a point

Choice of $q$ in Baby Rudin's Example 1.1

How many "super imaginary" numbers are there?

What do cones have to do with quadratics? Why is $2$ special?

Intuition behind normal subgroups

Geometric representation of Euler-Maclaurin Summation Formula

Why isn't $\int \frac{1}{x}~dx = \frac{x^0}{0}$?

Where is axiom of regularity actually used?

How to prove those "curious identities"?

What are the applications of finite calculus

On the vector spaces of Taylor Series and Fourier Series

Use of partial derivatives as basis vector

Differential forms without derivatives?

Why does partial differentiation give centre of a conic?

Discriminant of a Conic Section

Squaring both sides of an inequality of opposite signs

Simple proof that equilateral triangles have maximum area

Does $\sqrt{a+b} \le \sqrt a + \sqrt b$ hold for all positive real numbers a and b?

Equation of ellipse, hyperbola, parabola in complex form

Find $\int_0^1 \frac{f(x)}{\sqrt{1+x^2}}dx$

Reflection inside spherical mirror

What does Heron's formula naturally deform?

Show $\max{\{a,b\}}=\frac1{2}(a+b+|a-b|)$

Solve: $2^{\cos^{2014}x} - 2^{\sin^{2014} x} = \cos^{2013} (2x)$

A sum of series with the inverse squared central binomial coefficient

How to find this $\prod_{k=-\infty}^{+\infty}\frac{x^2+(4k+1-y)^2}{x^2+(4k+3-y)^2}$