# Euclid’s Elements: The first 4 axioms

```Euclid’s Elements:
The first 4 axioms
A modern form of the axioms.
1. For every point P and every point Q not equal to P
there exists a unique line that passes through P and Q.
2. For every segment AB and for every segment CD
there exists a unique point E such that B is between A
and E and segment CD is congruent to segment BE.
3. For every point O and every point A not equal to O
there exists a circle with center O and radius OA.
4. All right angles are congruent to each other.
Equivalent versions of the Parallel
Postulate
•
•
•
That, if a straight line falling on two straight
lines make the interior angle on the same side
less than two right angles, the two straight
lines, if produced indefinitely, meet on that side
on which are the angles less than two right
angles. (Euclid ca. 300BC)
For every line l and for every point P that does
not lie on l there exists a unique line m through
P that is parallel to l. (Playfair 1748-1819)
The sum of the interior angles in a triangle is
equal to two right angles. (Legendre 17521833 )
Girolamo Saccheri (1667-1733)
Only one of the following:
1.Hypothesis of right angle (HRA).
2.Hypothesis of Obtuse Angle (HOA)
3.Hypothesis of Acute Angle (HAA)
 way to study the parallel postulate.
HRA is equivalent to the parallel postulate.
Axioms 1-4 imply HOA not possible.
(1752-1833).
• In 1794 Legendre published El&eacute;ments de g&eacute;om&eacute;trie
which was the leading elementary text on the topic for
around 100 years.
• Always believed that the parallel postulate can be
deduced from the first four axioms, even after seeing
Bolyai’s proof. In 1832 (the year Bolyai published his
work on non-euclidean geometry) Legendre confirmed
his absolute belief in Euclidean space when he wrote:It is nevertheless certain that the theorem on the sum of the
three angles of the triangle should be considered one of those
fundamental truths that are impossible to contest and that are an
enduring example of mathematical certitude.
(1752-1833).
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• 1770 defended his thesis in mathematics
and physics at the Coll&egrave;ge Mazarin
• 1775 to 1780 he taught at &Eacute;cole Militaire
• 1782 won prize offered by the Berlin Academy for
treatise on projectiles
1791 member.
• Reappointed after Napoleon
• Quarreled with Gauss over priority of reciprocity and the
method of least squares.
• Worked in number theory, elliptic functions, geometry,
astronomy,…
• Refused to vote for the government candidate, lost his
pension and died in poverty.
(1752-1833).
• Gives correct refutation of HOA.
• Gives attempts of proofs for refutation of HAA.
• His attempt to show that the first four axioms
imply the parallel postulate leads to yet another
equivalent version of the parallel postulate:
Through any point in the interior of an angle it is
always possible to draw a line which meets both
sides of the angle.
```