Proposition I.6 If two angles in a triangle are equal the sides opposite these angles are also equal.

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3. Proposition I.6 is the converse of Thales' theorem about the base angles of an isoceles triangle being
equal which is Proposition I.5 of the Elements. Proposition I.6 is the first time in Elements that Euclid
uses proof by contradiction, reductio ad absurdum. Complete the following proof by showing that
ADBC is congruent to AACB which contradicts the fact that ADBC is inscribed within AACB.
As Euclid would put it the less cannot be equal to the greater. Technically this is a double reductio
ad absurdum argument and you must also show that the second case where AB < AC also leads to
a contradiction. You may finish this argument with the tried and true "it can be shown by a similar
argument that the assumption AB < AC also leads to a contradiction therefore..
Proposition I.6 If two angles in a triangle are equal the sides opposite these angles are also equal.
A
Proof Assume AABC has ZB = 2C but AB + AC.
Then either AB > AC or AB < AC. Assume AB >
AC. Then we construct a point D on AB so that
DB = AC.
B
Transcribed Image Text:3. Proposition I.6 is the converse of Thales' theorem about the base angles of an isoceles triangle being equal which is Proposition I.5 of the Elements. Proposition I.6 is the first time in Elements that Euclid uses proof by contradiction, reductio ad absurdum. Complete the following proof by showing that ADBC is congruent to AACB which contradicts the fact that ADBC is inscribed within AACB. As Euclid would put it the less cannot be equal to the greater. Technically this is a double reductio ad absurdum argument and you must also show that the second case where AB < AC also leads to a contradiction. You may finish this argument with the tried and true "it can be shown by a similar argument that the assumption AB < AC also leads to a contradiction therefore.. Proposition I.6 If two angles in a triangle are equal the sides opposite these angles are also equal. A Proof Assume AABC has ZB = 2C but AB + AC. Then either AB > AC or AB < AC. Assume AB > AC. Then we construct a point D on AB so that DB = AC. B
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