Practice (EndingDigits,TheDivideByNineMethod,MODBasic,EulerFermatTheorem,CRT,TheModMethod,SquareNumber)

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Let $$S_n=\sum_{k=1}^{2n}\frac{1}{n+k}=\frac{1}{n+1}+\frac{1}{n+2}+\cdots + \frac{1}{3n}$$

Does $\displaystyle\lim_{n\to\infty}S_n$ exist? If so, find its value. If not, prove the claim.


Show that $1-\cos{x} < x^2$ holds for all $x > 0$.


Determine whether the following series converge? $$\sum_{n=1}^{\infty}\left(1-\cos{\frac{\pi}{n}}\right)$$


Note that the point $(2, 1)$ is always on the curve $x^4 + ky^4 = 16+k$ regardless of the value of $k$. If for a particular non-zero value of $k$, $y'(2)=y''(2)$ along this curve. Find this $k$.


According to Newton’s law of cooling, the rate at which a cup of coffee cools is proportional to the difference between its temperature and that of the room it is in. A certain cup of coffee cools from $164^{\circ}$ to $140^{\circ}$ (all temperatures Fahrenheit) in five minutes, and then from $140^{\circ}$ to $122^{\circ}$ in the next five minutes. What is the temperature of the room?


It is well-known that the solution to the Fibonacci sequence is

$$F_n=\frac{1}{\sqrt{5}}\left(\left(\frac{1+\sqrt{5}}{2}\right)^n-\left(\frac{1-\sqrt{5}}{2}\right)^n\right)$$

Show that

$$\lim_{n\to\infty}\frac{F_{n+1}}{F_n}=\frac{1+\sqrt{5}}{2}$$


Compute $$\lim_{n\to\infty}\left(\sqrt{n+1}-\sqrt{n}\right)$$


Show that $$\lim_{n\to\infty}\int_0^1 x^n(1-x)^n dx = 0$$


Show the following result without explicitly performing the integration: $$\lim_{n\to\infty}\int_0^1(1-x^2)^ndx = 0$$


Show the following result without explicitly performing the integration $$\lim_{n\to\infty}\int_0^{\frac{\pi}{2}}\sin^n{x}dx$$


Without explicitly evaluating the integral, show that

$$\lim_{n\to\infty}\int_1^2\ln^n{x}dx =0\quad\text{and}\quad\lim_{n\to\infty}\int_2^3\ln^n{x}dx = \infty$$


Compute $$\int_0^{\frac{\pi}{4}}(\cos{x} - 2\sin{x}\sin(2x))dx$$


Let $f_0(x)=(\sqrt{e})^x$ , and recursively define $f_{n+1}(x) = f'_n(x)$ for integers $n\ge 0$. Compute $$\sum_{k=0}^{\infty}f_k(1)$$


Consider the parabola $y=ax^2 + 2019x + 2019$. There exists exactly one circle which is centered on the $x$-axis and is tangent to the parabola at exactly two points. It turns out that one of these tangent points is $(0, 2019)$. Find $a$.


What is the smallest natural number $n$ for which the following limit exists?

$$\lim_{x\to 0}\frac{\sin^nx}{\cos^2x(1-\cos{x})^3}$$


Turn the graph of $y=\frac{1}{x}$ by $45^{\circ}$ counter-clockwise and consider the bowl-like top part of the curve (the part above $y=0$). We let a $2D$ fluid accumulate in this $2D$ bowl until the maximum depth of the fluid is $\frac{2\sqrt{2}}{3}$. What’s the area of the fluid used?


Compute

$$\lim_{x\to 0}\frac{\frac{x^2}{2}+1-\sqrt{1+x^2}}{(\cos{x}-e^{x^2})\sin(x^2)}$$


Calculate $$\lim_{n\to\infty}\frac{1}{n^2}\sum_{k=1}^{n}\left(k\sin\frac{k\pi}{n}\right)$$


Find all the inflection points of $$\left\{\begin{array}{rl} x &=\cot {t} \\ y&=\frac{\cos(2t)}{\sin{t}} \end{array}\right., t\in(0, \pi)$$

Compute $$\int_0^4\frac{dx}{\sqrt{|x-2|}}$$


Compute $$\lim_{x\to 0}\frac{(1-\cos{x})^2}{x^2-x^2\cos^2{x}}$$


Compute $$\int_{-2}^{0}\frac{x^3 + 4x^2 + 7x -20}{x^2+4x+8}dx+\int_0^2\frac{2x^3-7x^2+9x-10}{x^2+4}dx$$


Calculuate $\displaystyle\lim_{x\to 0^+}x\ln{x}$.


Calculate $\displaystyle\lim_{x\to 0^+}x^x$.


Compute $$\lim_{n\to\infty}n^2\int_0^{\frac{1}{n}}x^{2018x+1}dx$$