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The report must be mathematically and linguistically well thought out and correct. Think about how you

use headings, different ways of writing formulas and so on. Any pictures must be made in a program and

not drawn by hand. Of course, it is not allowed to copy any text from the web or any book, but you must

think through and formulate your own

answers. All submitted texts are run in Urkund, which is a program that tracks plagiarism.

1. We denote the largest common divisor of the positive integers a1, a2, . . . , an with gcd(a1, a2, . . . , an). As in the case of

two numbers, this is the largest positive integer that divides all the ai

:na; for example, gcd(12, 28, 48) = 4. Do any of the

following implications apply?

gcd(a, b, c) = 1 ⇒ gcd(a, b) = gcd(a, c) = gcd(b, c) = 1 (1)

gcd(a, b) = gcd(a, c) = gcd(b, c) = 1 ⇒ gcd(a, b, c) = 1 (2)

¨Does the answer change if you know that a

2 + b

2 = c

2

? The answer must be justified, just yes

or no is not enough.

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2. Show that the set of rational numbers x such that x^2<2 has no minimum upper limit in Q, for example, by the

following steps: Assume that a^2<2 and define C by a^2+c=2. Show that a<3/2. Show that if 0<epsilon<c/4 and

epsilon<1, then (a+epsilon)^2<2.

3. Section 2.1 of Stillwell describes Euclid’s algorithm. Explain how that description is related to the one in the algebra

compendium in Mathematics I.

Give a proof of the division algorithm: Let a and b be two integers and b >0. Then there are unambiguously

determined numbers q and r such that a =qb+r and 0 ≤r <b. Let a and b be two quantities, for example lengths of

distances, and carry out Euclid’s operation which Stillwell writes about, that is, replace a, b with the pair a − b, b and so

on. Prove that this process ends if and only if a and b have a rational relationship.

4. It can be shown that the ratio between the diagonal and the side in a regular pentagon is the positive root of the

equation x

2 = x + 1. Use task 2 to show that the ratio (which is the so-called golden ratio) is not rational.

5. Verify your statement about the triangles in the parabola on page 10-11 (Figure 8) in Stillwell.

6. Infinity is a recurring theme in Stillwell’s book and in the context of mathematical philosophy a distinction is made

between current and potential infinity. What kind of infinity is it in

lim

x→∞ x

1

= 0 ?

Why?

7. Reflect on the calculations

Z ∞ dx

x

2

=

−

1

x

∞

= −

1

∞

−

1

1

= −(0 − 1) = 1

1 1

and

Z ∞

2

dx

x

2 + x − 2

=

1

3

Z ∞

2

1

x − 1

−

1

x + 2

dx

=

1

3

Z ∞

2

dx

x − 1

−

Z ∞

2

dx

x + 2

=

1

3

([log(x − 1)]∞

2 − [log(x + 2)]∞

2

)

=

1

3

(log ∞ − log 1 − (log ∞ − log 4))

=

1

3

(∞ − 0 − ∞ + log 4)

=

1

3

(∞ − ∞ + log 4)

=

1

3

log 4.

also

lim

x→∞

(x + 2 − x) = lim

x→∞

(x + 2) − limx→∞

x = ∞ − ∞ = 0

lim x→∞

(x + 2 − x) = lim x→∞

2 = 2.

8. Is area a property that an area has in itself or is it something we define, that is, assign it? How would Euclid

view this? Can you relate the discussion on page 10 in Stillwell to this question?

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9. Let b1, b2 and b3 be the lengths of the sides in a triangle and h1, h2 and h3 respectively heights. If one

assumes that the area of the triangle is a property of the triangle itself, then it follows that

b1h1

2

=

b2h2

2

=

b3h3

2

.

(3)

But if the area of a triangle is something we deny, then we have to show it in some other way (??). How do

you do that? Tip: Show that the triangles 4ADC and 4BEC are uniform.

10. Determine the conditions of the numbers a, b and c so that the line ax + by + c = 0 touches the

parabola y = x

2

. You can use derivatives, but I rate the solution higher if you provide a purely algebraic

solution. See Stillwell section 4.2 for an explanation of what I mean by an algebraic solution

11. There is plenty of more or less different evidence for Pythagoras’ theorem.

Here are two of them, which are said to originate from India.

In the first proof, only the left figure is used. The area of the large square can be written on the one hand

(a + b)

2

and on the other 4 · ab/2 + c

2

. So is

(a + b)

2 = 4 ·

ab

2

+ c

2

3

which is simplified to a

2 + b

2 = c

2

.In the second proof, both figures are used.

The squares have the same area and contain 4 copies of the triangle.

Thus the area of square C is equal to the area of A plus the area of B, which is Pythagoras’ theorem.

Which axioms and geometric theorems are used more or less implicitly in the evidence? (“Implicit” means unspoken,

that is, the sentences are used without being said.)

12. This task is about the so-called disk formula for the volume of a body in space (compare with Cavalieri’s

principle). A body lies along the x-axis between x = a and x = b. The area of a section perpendicular to the x-axis

at x is A(x).

An “in nitesimally thin” disk with thickness dx has the volume A(x) dx, so the body volume is

V =

Z b

a

A(x) dx.

Is it necessary to use infinitesimally thin slices in the proof or can you complete it without such? By the way, are ¨

there infinitesimally small quantities?

13. Two people play a simple dice game. A round consists of throwing the dice each time and winning at most (if

the players hit the same, they turn over). The one who has first won five rounds wins the game and the pot, ie

the money wagered. Suppose they have to interrupt the game before someone has won and they want to split

the pot fairly.

What can fairness mean in this context? Solve the problem with your definition of fairness if the players cancel

when one has won 4 times and the other 3 times.

14. The number of atoms of a radioactive substance which decomposes over a short period of time is proportional

to the total number of atoms. If the mass of the radioactive substance at time t ¨is m(t), so therefore

differentialekva-tionen m0

(t) = −λm(t), where λ is a positive constant that is characteristic of the substance

and is usually called the decay constant. Solve the equation and determine the relationship between the decay

constant and the half-life of the substance. What is the probability that a carbon-14 atom will decay within

1000 years if the half-life is 5730 years? Can you relate radioactive decay to any of the theories of probability?

NOTE: Answers to all sub-tasks. The solution should be the mathematical argumentation and

language lacks shortcomings.

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