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The intercept theorem, also known as Thales's theorem, basic proportionality theorem or side splitter theorem, is an important theorem in elementary geometry about the ratios of various line segments that are created if two rays with a common starting point are intercepted by a pair of parallels.
In geometry, Thales's theorem states that if A, B, and C are distinct points on a circle where the line AC is a diameter, the angle ∠ ABC is a right angle. Thales's theorem is a special case of the inscribed angle theorem and is mentioned and proved as part of the 31st proposition in the third book of Euclid 's Elements . [ 1 ]
Euclid's parallel postulate states: If a line segment intersects two straight lines forming two interior angles on the same side that sum to less than two right angles, then the two lines, if extended indefinitely, meet on that side on which the angles sum to less than two right angles. [13]
If the sum of the interior angles α and β is less than 180°, the two straight lines, produced indefinitely, meet on that side. In geometry, the parallel postulate, also called Euclid's fifth postulate because it is the fifth postulate in Euclid's Elements, is a distinctive axiom in Euclidean geometry.
Line art drawing of parallel lines and curves. In geometry, parallel lines are coplanar infinite straight lines that do not intersect at any point. Parallel planes are planes in the same three-dimensional space that never meet. Parallel curves are curves that do not touch each other or intersect and keep a fixed minimum distance. In three ...
As noted by Proclus, Euclid gives only three of a possible six such criteria for parallel lines. [5]: 309–310 [3]: Art. 89-90 Euclid's Proposition 29 is a converse to the previous two. First, if a transversal intersects two parallel lines, then the alternate interior angles are congruent.
A sequence of points and parallel lines is constructed as follows. The parallel line to AC through P 1 intersects AB in P 2 and the parallel line to BC through P 2 intersects AC in P 3. Continuing in this fashion the parallel line to AB through P 3 intersects BC in P 4 and the parallel line to AC through P 4 intersects AB in P 5.
The angle of parallelism, Φ, formulated as: (a) The angle between the x-axis and the line running from x, the center of Q, to y, the y-intercept of Q, and (b) The angle from the tangent of Q at y to the y-axis. This diagram, with yellow ideal triangle, is similar to one found in a book by Smogorzhevsky. [4]