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# Proofs

Proof by Mathematical Induction, Contradiction, Counterexample, Simple Deduction…

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##### Question 1

no calculator

easy

[Maximum mark: 4]

Consider two consecutive positive integers, $k$ and $k+1$.

Show that the difference of their squares is equal to the sum of the two integers.

easy

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##### Question 2

no calculator

easy

[Maximum mark: 4]

Prove that the sum of three consecutive positive integers is divisible by $3$.

easy

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##### Question 3

no calculator

easy

[Maximum mark: 4]

The product of three consecutive integers is increased by the middle integer.

Prove that the result is a perfect cube.

easy

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##### Question 4

no calculator

easy

[Maximum mark: 6]

1. Show that $(2n-1)^3 + (2n+1)^3 = 16n^3+12n$ for $n \in \mathbb{Z}$. 

2. Hence, or otherwise, prove that the sum of the cubes of any two consecutive odd integers is divisible by four. 

easy

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##### Question 5

no calculator

easy

[Maximum mark: 6]

Using mathematical induction, prove that $1^2 + 2^2 + \cdots + n^2 = \dfrac{n(n+1)(2n+1)}{6}$ for all $n \in \mathbb{Z}^+$.

easy

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##### Question 6

no calculator

easy

[Maximum mark: 7]

Use the principle of mathematical induction to prove that

\begin{aligned} \hspace{4em} 1\cdot2^1 + 2\cdot2^2 + 3\cdot2^3 + \cdots + n\cdot2^n &= 2 + (n-1)2^{n+1} \hspace{2em} \text{for all } n \in \mathbb{Z}^+. \\ \end{aligned}

easy

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##### Question 7

no calculator

easy

[Maximum mark: 6]

Let $r \in \mathbb{R}, r\neq 1$. Use the method of mathematical induction to prove that

\begin{aligned} \hspace{8.3em} 1+r+r^2+\cdots+r^n=\frac{1-r^{n+1}}{1-r} \hspace{2em} \text{for all } n\in \mathbb{Z}^+. \\ \end{aligned}

easy

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##### Question 8

no calculator

easy

[Maximum mark: 4]

Using the method of proof by contradiction, prove that $\sqrt{3}$ is irrational.

easy

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##### Question 9

calculator

easy

[Maximum mark: 4]

Prove by contradiction that $\log_4 7$ is an irrational number.

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##### Question 10

no calculator

easy

[Maximum mark: 7]

The Fibonacci sequence is defined as follows:

\begin{aligned} a_0 &= 0,\hspace{0.25em} a_1 = 1,\hspace{0.25em} a_2 = 1, \\[6pt] a_n &= a_{n-1}+a_{n-2} \hspace{0.5em}\text{for}\hspace{0.5em} n \geq 2. \qref{(FS)}\end{aligned}

Prove by mathematical induction that $a_1^2+a_2^2+\cdots+a_n^2=a_na_{n+1}$, where $n\in\mathbb{Z}^+$.

easy

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##### Question 11

no calculator

medium

[Maximum mark: 8]

Let $y = x^2 e^x$, for $x \in \mathbb{R}$.

1. Find $\dfrac{\mathrm{d}y}{\mathrm{d}x}$. 

2. Prove by mathematical induction that

$\hspace{4em} \dfrac{\mathrm{d}^n}{\mathrm{d}x^n}\big(x^2e^x\big) = \big(n(n-1) + 2nx + x^2\big)e^x \hspace{1.5em} \text{for all n \in \mathbb{Z}^+, n\geq2.}$



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##### Question 12

no calculator

medium

[Maximum mark: 9]

Let $f(x) = (x+1)e^{-2x}$, $x \in \mathbb{R}$.

1. Find $f'(x)$. 

2. Prove by induction that $\dfrac{\mathrm{d}^nf}{\mathrm{d}x^n} = \big[n(-2)^{n-1} + (-2)^n(x+1)\big]e^{-2x}$ for all $n \in \mathbb{Z}^+$.

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##### Question 13

no calculator

medium

[Maximum mark: 6]

Using the principle of mathematical induction, prove that $n(n^2+5)$ is divisible by $6$ for all integers $n \geq 1$.

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##### Question 14

calculator

hard

[Maximum mark: 8]

1. Solve the inequality $x^2 \geq 2x + 3$. 

2. Use mathematical induction to prove that $2^n > n^2 - 2$ for all $n \in \mathbb{Z}^+$, $n \geq 3$.

hard

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##### Question 15

calculator

hard

[Maximum mark: 8]

1. Solve the inequality $4x^2 \geq 4x + 2$. 

2. Use mathematical induction to prove that $3^n > 2n^2$ for all $n \in \mathbb{Z}^+$, $n \geq 2$.

hard

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##### Question 16

no calculator

hard

[Maximum mark: 14]

1. Show that $\dfrac{1}{2\sqrt{n+1}} < \sqrt{n+1} - \sqrt{n}$, where $n \in \mathbb{Z},\hspace{0.1em} n\geq 0$. 

2. Hence show that $\dfrac{1}{\sqrt{2}} < 2\sqrt{2} - 2$. 

3. Prove by mathematical induction that

$\hspace{4em} \sum_{r = 2}^n \dfrac{1}{\sqrt{r}} < 2\sqrt{n} - 2 \hspace{2em} \text{for all n \in \mathbb{Z}^+, n \geq 2.}$



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##### Question 17

no calculator

hard

[Maximum mark: 23]

Let $f(x) = (x-1)e^{\frac{x}{3}}$, for $x \in \mathbb{R}$.

1. Find $f'(x)$. 

2. Prove by induction that $\dfrac{\mathrm{d}^nf}{\mathrm{d}x^n} = \bigg(\dfrac{3n + x - 1}{3^n}\bigg)e^{\frac{x}{3}}$ for all $n \in \mathbb{Z}^+$. 

3. Find the coordinates of any local maximum and minimum points on the graph of $y = f(x)$. Justify whether such point is a maximum or a minimum. 

4. Find the coordinates of any points of inflexion on the graph of $y = f(x)$. Justify whether such point is a point of inflexion. 

5. Hence sketch the graph of $y = f(x)$, indicating clearly the points found in parts (c) and (d) and any intercepts with the axes. 

hard

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##### Question 18

no calculator

hard

[Maximum mark: 21]

1. Use de Moivre's theorem to find the value of $\left[\cos\left(\dfrac{\pi}{6}\right) + {\mathrm{\hspace{0.05em}i}\mkern 1mu}\sin \left(\dfrac{\pi}{6}\right)\right]^{12}$. 

2. Use mathematical induction to prove that

$\hspace{3.5em} (\cos \alpha - {\mathrm{\hspace{0.05em}i}\mkern 1mu}\sin \alpha)^n = \cos (n\alpha) - {\mathrm{\hspace{0.05em}i}\mkern 1mu}\sin (n\alpha) \hspace{1em} \text{for all } n \in \mathbb{Z}^+.$



Let $w = \cos \alpha + {\mathrm{\hspace{0.05em}i}\mkern 1mu}\sin \alpha$.

1. Find an expression in terms of $\alpha$ for $w^n - (w^\ast)^n$, $n \in \mathbb{Z}^+$, where $w^\ast$ is the complex conjugate of $w$. 

1. Show that $ww^\ast = 1$.

2. Write down and simplify the binomial expansion of $(w - w^\ast)^3$ in terms of $w$ and $w^\ast$.

3. Hence show that $\sin (3\alpha) = 3\sin \alpha - 4 \sin^3 \alpha$. 

2. Hence solve $4\sin^3\alpha + (2 \cos \alpha - 3) \sin \alpha = 0$ for $0 \leq \alpha \leq \pi$. 

hard

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##### Question 19

no calculator

hard

[Maximum mark: 21]

Let $f(x) = \dfrac{1}{\sqrt{1-x}}$,$x < 1$.

1. Show that $f''(x) = \dfrac{3}{4} (1-x)^{-5/2}$. 

2. Use mathematical induction to prove that

$f^{(n)}(x) = \left(\dfrac{1}{4}\right)^n \dfrac{(2n)!}{n!} (1-x)^{-1/2-n} \quad n\in \mathbb{Z},\enskip n\geq 2.$

Let $g(x)=\cos (mx)$, $m\in \mathbb{Q}$.

Consider the function $h$ defined by $h(x)=f(x) \times g(x)$ for $x<1$.

The $x^2$ term in the Maclaurin series for $h(x)$ has a coefficient of $-\dfrac{3}{4}$.

1. Find the possible values of $m$.

hard

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##### Question 20

no calculator

hard

[Maximum mark: 17]

The following diagram shows the graph of $y=\arctan(2x-3)+\dfrac{3\pi}{4}$ for $x\in \mathbb{R}$,
with asymptotes at $y=\dfrac{\pi}{4}$ and $y=\dfrac{5\pi}{4}$. 1. Describe a sequence of transformations that transforms the graph of
$y=\arctan x$ to the graph of $y=\arctan(2x-3)+\dfrac{3\pi}{4}$ for $x\in \mathbb{R}$.

2. Show that $\arctan p - \arctan q \equiv \arctan \left(\dfrac{p-q}{1+pq}\right)$.

3. Verify that $\arctan(x+2)-\arctan(x+1) = \arctan\left( \dfrac{1}{(x+1)^2+(x+1)+1}\right)$.

4. Using mathematical induction and the results from part (b) and (c), prove that

$\sum_{r=1}^n \arctan\left(\dfrac{1}{r^2+r+1}\right) = \arctan(n+1)-\dfrac{\pi}{4} \hspace{0.8em} \text{for } n\in \mathbb{Z}^{+}.$

hard

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##### Question 21

no calculator

easy

[Maximum mark: 6]

The first three terms of an arithmetic sequence are $u_1, 4u_1-9$, and $3u_1+18$.

1. Show that $u_1=9$. 

2. Prove that the sum of the first $n$ terms of this arithmetic sequence is a square number. 

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