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D.R.'s user avatar
D.R.'s user avatar
D.R.
  • Member for 7 years, 3 months
  • Last seen this week
13 votes

Demystifying the Determinant

8 votes

What is a good way to introduce Euler's totient function?

7 votes
Accepted

Why is "convexity" important in mathematics?

7 votes

Partial Fractions: Why does this shortcut method work?

7 votes
Accepted

Solve $f'(x)=f(x)^{2}+4$

7 votes

If $\frac{\cos x}{\cos y}+\frac{\sin x}{\sin y}=-1$, then what's the value of $4\left(\frac{\cos^3y}{\cos x}+\frac{\sin^3y}{\sin x}\right)$?

7 votes
Accepted

Rank of a matrix with entries $1, \dots, n^2$

5 votes

Calculate $\tan^2{\frac{\pi}{5}}+\tan^2{\frac{2\pi}{5}}$ without a calculator

5 votes

How to solve this ( and general )logarithmic Equation

5 votes
Accepted

How to take square root of $x^2 + y^2 = 144$?

5 votes
Accepted

Is every function eligible as a boundary condition?

5 votes
Accepted

Can Carathéodory's Extension Lemma be proven without use of the Carathéodory's Restriction Lemma?

4 votes
Accepted

If $f$ is continuous and differentiable, $f(0) = f'(0) = f'(1) = 0$, is there a tangent line to $f$ through the origin?

4 votes
Accepted

Power with complex exponents

4 votes
Accepted

A little confused about the Taylor series of $e^x$

4 votes

A 1400 years old approximation to the sine function by Mahabhaskariya of Bhaskara I

4 votes
Accepted

Prove $\bigcup_{n=1}^{\infty} [1/n^{2}, \infty) = (0, \infty)$

4 votes
Accepted

A box has three coins. One has two heads, another two tails and the last is a fair coin.

4 votes

Why $\sin\left( \frac 1 x \right) $ oscillates infinitely many times as $x \to 0$

3 votes
Accepted

Why does $\frac{d}{dx} a^x = \ln(a)a^x$ and the derivative of $x^n$ equals $nx^{x-1}$

3 votes
Accepted

When doesn't a differential equation have a solution of the form $Ae^{at}$

3 votes

What is wrong in my solution attempt to the $80^\circ$-$80^\circ$-$20^\circ$ problem?

3 votes
Accepted

Find a + b + c + bc.

3 votes

If $\sin x = a$, for $x$ in the Second Quadrant, then $\tan x = \text{???}$

3 votes

Is there an elementary proof that $\sum \limits_{k=1}^n \frac1k$ is never an integer?

3 votes
Accepted

Clever integral and formula

3 votes

Proving that the ratio of the hypotenuse of an isosceles right triangle to the leg is irrational

3 votes
Accepted

Asymptotic of $\sum (H_n-H_{k-1})\ln(k)$ where $H_k$ are the harmonic numbers

3 votes

Interpreting tail events and their "dependence" on finite r.v.'s

3 votes

How can we show that all square function are absolute integrable

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