More challenging Questions
Induction 1
- Factorials are defined inductively by the rule 0!=1and(n+1)!=n!×(n+1). Then binomial coefficients are defined for 0≤k≤n by \binom{n}{k}=\frac{n!}{k!(n-k)!}. Prove from these definitions that \binom{n}{k}+\binom{n}{k+1}=\binom{n+1}{k+1}, and deduce the Binomial Theorem: that for any x and y, (x+y)^n=\sum_{k=0}^{n}\binom{n}{k}x^ky^{n-k}.
- Prove that \sum_{r=1}^n \frac{1}{r^2}\leq 2-\frac{1}{n}.
- Prove that for n=1,2,3,\dots \sqrt{n}\leq \sum_{k=1}^{n}\frac{1}{\sqrt{k}}\leq 2\sqrt{n}-1.
- Let \displaystyle A=\left(\begin{matrix} 5&-1\\4&1 \end{matrix}\right). Show that A^n=3^{n-1}\left(\begin{matrix} 2n+3&-n\\4n&3-2n \end{matrix}\right) for n=1,2,3,\dots. Can you find a matrix B such that B^2=A?
- Let k be a positive integer. Prove by induction on n that \sum_{r=1}^n r(r+1)(r+2)\dots (r+k-1)=\frac{n(n+1)(n+2)\dots(n+k)}{k+1}. Show now by induction on k that \sum_{r=1}^n r^k = \frac{n^{k+1}}{k+1}+E_k(n) where E_k(n) is a polynomial of degree at most k.
Induction 2
- Show that n lines in the plane, no two of which are parallel and no three meeting in a point, divide the plane into \frac{n^2+n+2}{2} regions.
- Prove that for every positive integer n, that 3^{3n-2}+2^{3n+1} is divisible by 19.
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- Show that if u^2-2v^2=1 then (3u+4v)^2-2(2u+3v)^2=1.
- Beginning with u_0=3, v_0=2, show that the recursionu_{n+1}=3u_n+4v_n \quad \text{and} \quad v_{n+1}=2u_n+3v_ngenerates infinitely many integer pairs (u,v) which satisfy u^2-2v^2=1.
- How can this process be used to produce better and better rational approximations to \sqrt{2}? How many times need this process be repeated to produce a rational approximation accurate to 6 decimal places?
- The Fibonacci numbers F_n are defined by the recurrence relationF_n=F_{n-1}+F_{n-2}\quad \text{for $n\geq 2$}and F_0=0 and F_1=1.Prove that for every integer n\geq 0, thatF_n=\frac{\alpha^n-\beta^n}{\sqrt{5}}where\alpha=\frac{1+\sqrt{5}}{2},\quad \text{and}\quad \beta=\frac{1-\sqrt{5}}{2}.(Hint: you may find it helpful to show first that the two roots of the equation x^2=x+1 are \alpha and \beta.)
- The sequence of numbers x_0, x_1, x_2, \dots begins with x_0=1 and x_1=1 and is then recursively determined by the equationsx_{n+2}=4x_{n+1}-3x_n+3^n\quad \text{for $n\geq 0$}.
- Find the values of x_2, x_3, x_4, and x_5.
- Can you find a solution of the formx_n=A+B\times 3^n +C \times n 3^nwhich agrees with the values of x_0,\dots,x_5 that you have found?
- Use induction to prove that this is the correct formula for x_n for all n\geq 0.
Algebra 1
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- Find the remainder when n^2+4 is divided by 7 for 0\leq n < 7.
Deduce that n^2+4 is not divisible by 7, for every positive integer n. (Hint: write n=7k+r where 0\leq r<7.) - Now k is an integer such that n^3+k is not divisible by 4 for all integers n. What are the possible values of k?
- Find the remainder when n^2+4 is divided by 7 for 0\leq n < 7.
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- Prove that if a, b are positive real numbers then\sqrt{ab}\leq \frac{1}{2}(a+b).
- Now let a_1, a_2, \dots, a_n be positive real numbers. Let S=a_1+a_2+\dots+a_n and P=a_1a_2\cdots a_n. Suppose that a_i and a_j are distinct. Show that replacing a_i and a_j with (a_i+a_j)/2 and (a_i+a_j)/2 increases P without changing S.
Deduce that(a_1a_2\cdots a_n)^{1/n}\leq \frac{a_1+a_2+\cdots+a_n}{n}.
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- Let n be a positive integer. Show thatx^n-y^n=(x-y)(x^{n-1}+x^{n-2}y+\cdots + xy^{n-2}+y^{n-1}).
- Let a also be a positive integer. Show that if a^n-1 is prime then a=2 and n is prime.
Is it true that if n is prime then 2^n-1 is also prime?
- Let a, b, r, s be rational numbers with s\neq 0. Suppose that the number r+s\sqrt{2} is a root of the quadratic equationx^2+ax+b=0.Show that r-s\sqrt{2} is also a root.
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- The cubic equation ax^3+bx^2+cx+d=0 has roots \alpha, \beta, \gamma and so factorises asa(x-\alpha)(x-\beta)(x-\gamma).Determine\alpha+\beta+\gamma, \quad \alpha\beta+\beta\gamma+\gamma\alpha, \quad \alpha\beta\gamma,in terms of a, b, c, d. What does \alpha^2+\beta^2+\gamma^2 equal?
- Show that \cos 3\theta=4\cos^3 \theta - 3 \cos \theta.
- By considering the roots of the equation 4x^3-3x-\cos 3\theta=0 deduce that \cos \theta \cos(\theta+2\pi/3)\cos(\theta+4\pi/3)=\frac{\cos(3\theta)}{4}.What do\cos\theta+\cos(\theta+2\pi/3)+\cos(\theta+4\pi/3)\quad \text{and}\quad \cos^2\theta+\cos^2(\theta+2\pi/3)+\cos^2(\theta+4\pi/3)equal?
Algebra 2
- Under what conditions on the real numbers a, b, c, d, e, f do the simultaneous equationsax+by=e\quad \text{and}\quad cx+dy=fhave (a) a unique solution, (b) no solution, (c) infinitely many solutions in x and y.
Select values of a, b, c, d, e, f for each of these cases, and sketch on separate axes the lines ax+by=e and cx+dy=f. - For what values of a do the simultaneous equations\begin{align*}x+2y+a^2z&=0,\\x+ay+z&=0,\\x+ay+a^2z&=0,\end{align*}have a solution other than x=y=z=0? For each such a find the general solution to the above equations.
- Do 2\times 2 matrices exist satisfying the following properties? Either find such matrices or show that no such exist.
- A such that A^5=I and A^i\neq I for 1\leq i\leq 4,
- A such that A^n\neq I for all positive integers n,
- A and B such that AB\neq BA,
- A and B such that AB is invertible and BA is singular (i.e. not invertible)
- A such that A^5=I and A^{11}=0.
- LetA=\left(\begin{matrix}a&b\\c&d\end{matrix}\right)\quad \text{and let}\quad A^T=\left(\begin{matrix}a&c\\b&d\end{matrix}\right)be a 2\times2 matrix and its transpose. Suppose that \det A=1 andA^TA=\left(\begin{matrix}1&0\\0&1\end{matrix}\right).Show that a^2+c^2=1, and hence that a and c can be written asa=\cos \theta \quad\text{and}\quad c=\sin \thetafor some \theta in the range 0\leq \theta < 2\pi. Deduce that A has the formA=\left(\begin{matrix}\cos\theta&-\sin\theta\\ \sin\theta & \cos\theta\end{matrix}\right).
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- Prove that\det(AB)=\det(A)\det(B)for any 2\times2 matrices A and B.
- Let A denote the 2\times 2 matrix\left(\begin{matrix}a&b\\c&d\end{matrix}\right).Show that\begin{equation}A^2-(\operatorname{trace}A)A+(\det A)I=0 \qquad \qquad \text{(1)}\end{equation}where
- \operatorname{trace}A=a+d is the trace of A, that is the sum of the diagonal elements,
- \det A=ad-bc is the determinant of A,
- I is the 2\times 2 identity matrix.
- Suppose now that A^n=0 for some n\geq 2. Prove that \det A=0. Deduce using equation (1) that A^2=0.
Calculus 1
- Sketch the graph of the curvey=\frac{x^2+1}{(x-1)(x-2)}carefully labelling any turning points and asymptotes.
- The parabola x=y^2+ay+b crosses the parabola y=x^2 at (1,1) making right angles.
Calculate the values of a and b.
On the same axes, sketch the two parabolas. - The curve C in the xy-plane has equationx^2+xy+y^2=1.By solving \displaystyle \frac{\mathrm{d}y}{\mathrm{d}x}=0, show that the maximum and minimum values taken by y are \displaystyle \pm \frac{2}{\sqrt{3}}.
By changing to polar co-ordinates (x=r\cos\theta, y=r\sin\theta), sketch the curve C.
What is the greatest distance of a point on C from the origin? - Sketch the curve y=x^3+ax+b for a selection of values of a and b.
Suppose now that a is negative. Find the co-ordinates of the turning points of the graph and deduce that y=0 has exactly two roots whenb=\pm\frac{2a}{3}\sqrt{\frac{-a}{3}}For what values of b does the equation y=0 have three distinct real roots? - On separate xu- and yu-axes sketch the curves u=8(x^3-x) and u=e^y/y labelling all turning points.
[Harder] Hence sketch the curve e^y=8y(x^3-x).
Calculus 2
- Use calculus, or trigonometric identities, to prove the following inequalities for \theta in the range 0<\theta<\frac{\pi}{2};
- \sin\theta<\theta,
- \theta<\tan\theta,
- \cos 2\theta < \cos^2\theta.
Hence, without directly calculating the following integrals, rank them in order of size.\text{(a)}\quad \int_0^1 x^3\cos x \,\mathrm{d}x,\qquad\text{(b)}\quad \int_0^1 x^3\cos^2x \,\mathrm{d}x,\qquad\text{(c)}\quad \int_0^1 x^2\sin x \cos x \,\mathrm{d}x,\qquad\text{(d)}\quad \int_0^1 x^3 \cos 2x \,\mathrm{d}x.
- Show that the equation\sin x=\frac{1}{2}xhas three roots. Using Newton-Raphson, or a similar numerical method, find the positive root to 6 d.p.
The equation \sin x =\lambda x has three real roots when \lambda=\alpha or when \beta<\lambda<1 for two real numbers \alpha<0<\beta. Plot, on the same axes, the curvesy=\sin x,\qquad y=\alpha x,\qquad y=\beta x. - Let S denote the circle in the xy-plane with centre (0,0) and radius 1. A regular m-sided polygon I_m is inscribed in S and a regular n-sided polygon C_n is circumscribed about S.
- By considering the perimeter of I_m and the area bounded by C_n, prove thatm\sin\left(\frac{\pi}{m}\right)<\pi < n \tan \left(\frac{\pi}{n}\right)for all natural numbers m,n\geq 3.
- Archimedes showed (using this method) that 3\frac{10}{71}<\pi <3 \frac{1}{7}. What are the smallest values of m and n needed to verify Archimedes' inequality?
- Find the coefficients of 1, x, x^2, x^3, x^4 in the power series expansion (Taylor's series expansion) for f(x)=\sec x.
Use this approximation to make an estimate for \sec\frac{1}{10}. With the aid of a calculator, find to how many decimal places the approximation is accurate. - Show that \int \ln x \,\mathrm{d}x=x\ln x - x +\text{constant}.
Sketch the graph of the equation y=\ln x. By consideration of areas on your graph, show that n\ln n -n+1<\sum_1^n \ln r < (n+1)\ln (n+1)-n
Let G_n=\sqrt[n]{n!} denote the geometric mean of 1, 2, \dots, n. Show that G_n/n approaches 1/e as n becomes large.
Calculus 3
- Evaluate\int\frac{\ln x}{x}\,\mathrm{d}x,\qquad \int x \sec^2 x \,\mathrm{d}x, \qquad \int_3^\infty \frac{\mathrm{d}x}{(x-1)(x-2)},\qquad \int_0^1 \tan^{-1}x\,\mathrm{d}x,\qquad \int_0^1 \frac{\mathrm{d}x}{e^x+1}.
- Evaluate, using trigonometric and/or hyperbolic substitutions,\int \frac{\mathrm{d}x}{x^2+1},\qquad \int_1^2 \frac{\mathrm{d}x}{\sqrt{x^2-1}}, \qquad \int \frac{\mathrm{d}x}{\sqrt{4-x^2}},\qquad \int_2^\infty\frac{\mathrm{d}x}{(x^2-1)^{3/2}}.
- By completing the square in the denominator, and using the substitutionx=\frac{\sqrt{2}}{3}\tan \theta -\frac{1}{3}evaluate\int\frac{\mathrm{d}x}{3x^2+2x+1}.By similarly completing the square in the following denominators, and making appropriate trigonometric and/or hyperbolic substitutions, evaluate the following integrals\int\frac{\mathrm{d}x}{\sqrt{x^2+2x+5}},\qquad \int_0^\infty \frac{\mathrm{d}x}{4x^2+4x+5}.
- Let t=\tan \frac{1}{2}\theta. Show that\sin\theta =\frac{2t}{1+t^2},\qquad \cos\theta=\frac{1-t^2}{1+t^2},\qquad \tan \theta = \frac{2t}{1-t^2}and that\mathrm{d}\theta=\frac{2\mathrm{d}t}{1+t^2}.Use the substitution t=\tan \frac{1}{2}\theta to evaluate\int_0^{\pi/2}\frac{\mathrm{d}\theta}{(1+\sin\theta)^2}.
- LetI_n=\int_0^{\pi/2} x^n\sin x \,\mathrm{d}x.
Evaluate I_0 and I_1.Show, using integration by parts, thatI_n=n\left(\frac{\pi}{2}\right)^{n-1}-n(n-1)I_{n-2}.Hence, evaluate I_5 and I_6.
Calculus 4
- Find the general solutions of the following separable differential equations.\frac{\mathrm{d}y}{\mathrm{d}x}=\frac{x^2}{y}, \qquad\frac{\mathrm{d}y}{\mathrm{d}x}=\frac{\cos^2x}{\cos^2 2y},\qquad\frac{\mathrm{d}y}{\mathrm{d}x}=e^{x+2y}.
- Find the solution of the following initial value problems. On separate axes sketch the solution to each problem.
\begin{align*}\frac{\mathrm{d}y}{\mathrm{d}x}&=\frac{1-2x}{y},\qquad y(1)=-2,\\\frac{\mathrm{d}y}{\mathrm{d}x}&=\frac{x(x^2+1)}{4y^3},\qquad y(0)=\frac{-1}{\sqrt{2}},\\\frac{\mathrm{d}y}{\mathrm{d}x}&=\frac{1+y^2}{1+x^2},\qquad y(0)=1.\end{align*} - The equation for Simple Harmonic Motion, with constant frequency \omega, is\frac{\mathrm{d}^2x}{\mathrm{d}t^2}=-\omega^2 x.Show that\frac{\mathrm{d}^2x}{\mathrm{d}t^2}=v\frac{\mathrm{d}v}{\mathrm{d}x}where \displaystyle v=\frac{\mathrm{d}x}{\mathrm{d}t} denotes velocity. Find and solve a separable differential equation in v and x given that x=a when v=0.
Hence show thatx(t)=a\sin\left(\omega t+\varepsilon\right) for some constant \varepsilon. - Find the most general solution of the following homogenous constant coefficient differential equations:
\begin{align*}\frac{\mathrm{d}^2y}{\mathrm{d}x^2}-y&=0,\\\frac{\mathrm{d}^2y}{\mathrm{d}x^2}+4y&=0,\quad\text{where $y(0)=y'(0)=1$.}\\\frac{\mathrm{d}^2y}{\mathrm{d}x^2}+3\frac{\mathrm{d}y}{\mathrm{d}x}+2y&=0,\\\frac{\mathrm{d}^2y}{\mathrm{d}x^2}-4\frac{\mathrm{d}y}{\mathrm{d}x}+4y&=0,\quad\text{where $y(0)=y'(0)=1$.}\end{align*} - Write the differential equation(2x+y)+(x+2y)\frac{\mathrm{d}y}{\mathrm{d}x}=0in the form\frac{\mathrm{d}}{\mathrm{d}x}\left(F(x,y)\right)=0where F(x,y) is a polynomial in x and y. Hence find the general solution of the equation.
Use this method to find the general solution of\left(y\cos x +2xe^y\right)+\left(\sin x +x^2e^y-1\right)\frac{\mathrm{d}y}{\mathrm{d}x}=0.
Calculus 5
- Find all solutions of the following separable differential equations:\begin{align*}\frac{\mathrm{d}y}{\mathrm{d}x}&=\frac{y-xy}{xy-x}. \\\frac{\mathrm{d}y}{\mathrm{d}x}&=\frac{\sin^{-1}x}{y^2\sqrt{1-x^2}},\quad y(0)=0. \\\frac{\mathrm{d}^2y}{\mathrm{d}x^2}&=(1+3x^2)\left(\frac{\mathrm{d}y}{\mathrm{d}x}\right)^2\quad\text{where $y(1)=0$ and $y'(1)=\frac{-1}{2}$.}\end{align*}
- Use the method of integrating factors to solve the following equations with initial conditions
\begin{align*} \frac{\mathrm{d}y}{\mathrm{d}x}+xy &= x,\quad \text{where $y(0)=0$}.\\ 2x^3 \frac{\mathrm{d}y}{\mathrm{d}x} -3x^2y &=1,\quad \text{where $y(1)=0$}.\\ \frac{\mathrm{d}y}{\mathrm{d}x} - y\tan x &=1,\quad \text{where $y(0)=1$}. \end{align*} - Find the most general solution of the following inhomogeneous constant coefficient differential equations:
\begin{align*} \frac{\mathrm{d}^2y}{\mathrm{d}x^2}+3\frac{\mathrm{d}y}{\mathrm{d}x}+2y &= x,\\ \frac{\mathrm{d}^2y}{\mathrm{d}x^2}+3\frac{\mathrm{d}y}{\mathrm{d}x}+2y &= \sin x,\\ \frac{\mathrm{d}^2y}{\mathrm{d}x^2}+3\frac{\mathrm{d}y}{\mathrm{d}x}+2y &= e^x,\\ \frac{\mathrm{d}^2y}{\mathrm{d}x^2}+3\frac{\mathrm{d}y}{\mathrm{d}x}+2y &= e^{-x}. \end{align*} -
- By making the substitution y(x)=xv(x) in the following homogeneous polar equations, convert them into separable differential equation involving v and x, which you should then solve.
\begin{align*} \frac{\mathrm{d}y}{\mathrm{d}x}&=\frac{x^2+y^2}{xy}.\\ x\frac{\mathrm{d}y}{\mathrm{d}x}&=y+\sqrt{x^2+y^2}. \end{align*} - Make substitutions of the form x=X+a, y=Y+b, to turn the differential equation\frac{\mathrm{d}y}{\mathrm{d}x}=\frac{x+y-3}{x-y-1}into a homogeneous polar differential equation in X and Y. Hence find the general solution of the above equation.
- By making the substitution y(x)=xv(x) in the following homogeneous polar equations, convert them into separable differential equation involving v and x, which you should then solve.
- A particle P moves in the xy-plane. Its co-ordinates x(t) and y(t) satisfy the equations\frac{\mathrm{d}y}{\mathrm{d}t}=x+y\quad\text{and}\quad \frac{\mathrm{d}x}{\mathrm{d}t}=x-y,and at time t=0 the particle is at (1,0). Find, and solve, a homogeneous polar equation relating x and y.
By changing to polar co-ordinates (r^2=x^2+y^2, \tan\theta=y/x), sketch the particle's journey for t\geq 0.
Complex Numbers
- By writing \omega= a +ib (where a and b are real), solve the equation\omega^2=-5-12i.Hence find the two roots of the quadratic equationz^2-(4+i)z+(5+5i)=0.
- By substituting z=x+iy or z=re^{i\theta} into the following equations and inequalities, sketch the following regions of the complex plane on separate Argand diagrams:
- |z-3-4i|<5
- \displaystyle \arg(z)=\frac{\pi}{3}
- \displaystyle 0\leq \operatorname{Re}\left(\frac{iz+3}{2}\right)<2
- e^z=1
- \operatorname{Im}(z^2)<0
- Find the image of the point z=2+it under each of the following transformations.
- z\mapsto iz
- z\mapsto z^2
- z\mapsto e^z
- \displaystyle z\mapsto \frac{1}{z}
By letting t vary over all real values find the image of the line \operatorname{Re}z=2 under the same transformations.
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- Given that e^{i\theta}=\cos \theta + i\sin \theta, prove that\cos(\alpha+\beta)=\cos \alpha\cos\beta - \sin\alpha \sin \beta.
- Use De Moivre's Theorem to show that\cos 5\theta = 16 \cos^5 \theta -20 \cos^3 \theta +5 \cos \theta.
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- Let z=\cos\theta+i\sin\theta and let n be an integer. Show that2\cos\theta = z+\frac{1}{z}\quad \text{and that} \quad 2i\sin\theta=z-\frac{1}{z}.Find expressions for \cos n\theta and \sin n\theta in terms of z.
- Show that\cos^5 \theta = \frac{1}{16}\left(\cos 5 \theta + 5 \cos 3 \theta +10\cos \theta \right)and hence find \int_0^{\pi/2}\cos^5 \theta \,\mathrm{d}\theta.
Geometry
- Describe the regions of space given by the following vector equations. In each, \mathbf{r} denotes the vector x\mathbf{i}+y\mathbf{j}+z\mathbf{k}; \cdot and \wedge denote the scalar (dot) and vector (cross) product:
- \mathbf{r}\wedge\left(\mathbf{i}+\mathbf{j}\right)=\left(\mathbf{i}-\mathbf{j}\right),
- \mathbf{r}\cdot \mathbf{i}=1,
- |\mathbf{r}-\mathbf{i}|=|\mathbf{r}-\mathbf{j}|,
- |\mathbf{r}-\mathbf{i}=1,
- \mathbf{r}\cdot \mathbf{i}=\mathbf{r}\cdot \mathbf{j}=\mathbf{r} \cdot \mathbf{k},
- \mathbf{r}\wedge\mathbf{i}=\mathbf{i}.
- Find the shortest distance between the lines\frac{x-1}{2}=\frac{y-3}{3}=\frac{z}{2}\qquad\text{and}\qquad x=2,\quad \frac{y-1}{2}=z(Hint: parametrise the lines and write down the vector between two arbitrary points on the lines; then determine when this vector is normal to both lines.)
- Let L_\theta denote the line through (a,b) making an angle \theta with the x-axis. Show that L_\theta is a tangent of the parabola y=x^2 if and only if\tan^2\theta - 4a \tan \theta +4b=0.(Hint: parametrise L_\theta as x=a+\lambda \cos\theta and y=b+\lambda \sin \theta and determine when L_\theta meets the parabola precisely once.)
Show that the tangents from (a,b) to the parabola subtend an angle of \pi/4 if and only if1+24b+16b^2=16a^2.(Hint: use the formula \tan(\theta_1-\theta_2)=(\tan\theta_1-\tan\theta_2)/(1+\tan\theta_1\tan\theta_2).)
Sketch the curve 1+24y+16y^2=16x^2 and the original parabola on the same axes. - What transformations of the xy-plane do the following matrices represent?
- \left(\begin{matrix}x\\y\end{matrix}\right)\mapsto \left(\begin{matrix}1&0\\0&-1 \end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)
- \left(\begin{matrix} x\\y \end{matrix}\right)\mapsto \left(\begin{matrix} 2&0\\0&1 \end{matrix}\right)\left(\begin{matrix} x\\y \end{matrix}\right)
- \left(\begin{matrix} x\\y \end{matrix}\right)\mapsto \left(\begin{matrix} 1/2&1/2\\1/2&1/2 \end{matrix}\right)\left(\begin{matrix} x\\y \end{matrix}\right)
- \left(\begin{matrix} x\\y \end{matrix}\right)\mapsto \left(\begin{matrix} \cos\theta&-\sin\theta\\ \sin\theta&\cos\theta \end{matrix}\right)\left(\begin{matrix} x\\y \end{matrix}\right)
Which, if any, of these transformations are invertible?
- The cycloid is the curve given parametrically by the equationsx(t)=t-\sin t, \quad \text{and}\quad y(t)=1-\cos t \quad \text{for $0\leq t\leq 2\pi$}.
- Sketch the cycloid.
- Find the arc-length of the cycloid.
- Find the area bounded by the cycloid and the x-axis.
- Find the area of the surface of revolution generated by rotating the cycloid around the x-axis.
- Find the volume enclosed by the surface of revolution generated by rotating the cycloid around the x-axis.