Standard formula: \(\displaystyle\int\frac{dx}{\sqrt{a^2-x^2}}=\arcsin\!\left(\frac{x}{a}\right)+C\) with \(a=2\).
Answer: \(\arcsin\!\left(\dfrac{x}{2}\right)+C\).
Factor: \(x^2-x-2=(x-2)(x+1)\). Partial fractions: \(\dfrac{2x+1}{(x-2)(x+1)}=\dfrac{A}{x-2}+\dfrac{B}{x+1}\).
Multiply both sides by \((x-2)(x+1)\): \(2x+1=A(x+1)+B(x-2)\).
\(x=2\): \(5=3A\Rightarrow A=\frac{5}{3}\). \(x=-1\): \(-1=-3B\Rightarrow B=\frac{1}{3}\).
\(\displaystyle\int=\frac{5}{3}\ln|x-2|+\frac{1}{3}\ln|x+1|+C\).
5
Improper IntegralsMedium
Does \(\displaystyle\int_1^\infty \frac{1}{x^2}\,dx\) converge? If so, find its value.
A
Diverges
B
Converges to \(2\)
C
Converges to \(1\)
D
Converges to \(\dfrac{1}{2}\)
✓ Solution
\(\displaystyle\int_1^\infty x^{-2}\,dx = \lim_{t\to\infty}\left[-\frac{1}{x}\right]_1^t = \lim_{t\to\infty}\!\left(-\frac{1}{t}+1\right)=1\).
The integral converges to \(1\).
Sequences & Series
Convergence Tests & Series
Geometric: \(\sum_{n=0}^\infty ar^n = \dfrac{a}{1-r}\) when \(|r|<1\)
Ratio Test: \(L=\lim_{n\to\infty}\left|\dfrac{a_{n+1}}{a_n}\right|\). Converges if \(L<1\), diverges if \(L>1\)
Alternating Series Test (Leibniz): converges if \(b_n\searrow0\)
Divergence Test: if \(\lim a_n\neq 0\), series diverges
6
Geometric SeriesMedium
Find the sum of the series \(\displaystyle\sum_{n=0}^{\infty}\left(\frac{2}{3}\right)^n\).
A
\(2\)
B
\(3\)
C
\(\dfrac{2}{3}\)
D
Diverges
✓ Solution
Geometric series with \(a=1\) and \(r=\frac{2}{3}\). Since \(|r|<1\), it converges.
Sum \(=\dfrac{a}{1-r}=\dfrac{1}{1-\frac{2}{3}}=\dfrac{1}{\frac{1}{3}}=3\).
7
Ratio TestHard
Determine the convergence of \(\displaystyle\sum_{n=1}^{\infty}\frac{n!}{3^n}\).
A
Converges by Ratio Test
B
Diverges by Ratio Test
C
Converges by p-Series
D
Ratio Test is inconclusive
✓ Solution
\(\dfrac{a_{n+1}}{a_n}=\dfrac{(n+1)!}{3^{n+1}}\cdot\dfrac{3^n}{n!}=\dfrac{n+1}{3}\to\infty\) as \(n\to\infty\).
Since \(L=\infty>1\), the series diverges by the Ratio Test.
8
Alternating SeriesMedium
Does \(\displaystyle\sum_{n=1}^{\infty}\frac{(-1)^n}{n}\) converge?
A
Diverges
B
Converges absolutely
C
Converges conditionally
D
Diverges by Divergence Test
✓ Solution
This is the alternating harmonic series. By the Alternating Series Test: \(b_n=\frac{1}{n}\) is decreasing and \(\lim_{n\to\infty}\frac{1}{n}=0\), so it converges.
But \(\sum\frac{1}{n}\) (harmonic series) diverges, so convergence is conditional, not absolute.
Radius of Convergence: \(R=\lim_{n\to\infty}\left|\dfrac{a_n}{a_{n+1}}\right|\)
\(\dfrac{1}{1-x}=\displaystyle\sum_{n=0}^\infty x^n\) for \(|x|<1\)
10
Radius of ConvergenceHard
Find the radius of convergence of \(\displaystyle\sum_{n=0}^{\infty}\frac{x^n}{n+1}\).
A
\(R=0\)
B
\(R=\infty\)
C
\(R=1\)
D
\(R=2\)
✓ Solution
Ratio Test: \(L=\lim_{n\to\infty}\left|\dfrac{x^{n+1}}{n+2}\cdot\dfrac{n+1}{x^n}\right|=|x|\cdot\lim_{n\to\infty}\dfrac{n+1}{n+2}=|x|\).
Converges when \(|x|<1\). Therefore \(R=1\).
If \(x=t^2\) and \(y=t^3\), find \(\dfrac{dy}{dx}\).
A
\(\dfrac{3t}{2}\)
B
\(\dfrac{2}{3t}\)
C
\(3t^2\)
D
\(\dfrac{3t^2}{2t}=\dfrac{3t}{2}\) (same as A)
✓ Solution
\(\dfrac{dx}{dt}=2t\), \(\dfrac{dy}{dt}=3t^2\).
\(\dfrac{dy}{dx}=\dfrac{3t^2}{2t}=\dfrac{3t}{2}\). Note: options A and D are identical — the answer is \(\dfrac{3t}{2}\).
14
Polar AreaHard
Find the area enclosed by \(r=2\cos\theta\) (one full loop).
A
\(\pi\)
B
\(2\pi\)
C
\(4\)
D
\(\dfrac{\pi}{2}\)
✓ Solution
\(r=2\cos\theta\) is a circle of radius 1. Area \(=\pi r^2=\pi\cdot 1^2=\pi\).
Via formula: \(A=\frac{1}{2}\int_{-\pi/2}^{\pi/2}(2\cos\theta)^2\,d\theta=2\int_{-\pi/2}^{\pi/2}\cos^2\theta\,d\theta=2\cdot\frac{\pi}{2}=\pi\).