Geometry with isosceles triangles

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SUMMARY

The discussion centers on proving that line segment CD is perpendicular to base AB in the context of two isosceles triangles, ABC and ABD, which share the same base but exist in different planes. Participants emphasize the importance of defining perpendicularity, noting that traditional definitions involve right angles formed by intersecting lines. A key point raised is the necessity of considering projections of lines onto planes to establish perpendicular relationships, particularly when dealing with skew lines.

PREREQUISITES
  • Understanding of geometric principles, specifically properties of isosceles triangles.
  • Familiarity with the concept of perpendicular lines in three-dimensional space.
  • Knowledge of projections of lines onto planes.
  • Basic understanding of geometric definitions and their implications.
NEXT STEPS
  • Research the properties of isosceles triangles in three-dimensional geometry.
  • Study the definition and properties of perpendicular lines in Euclidean space.
  • Explore the concept of line projections onto planes and their geometric significance.
  • Investigate the implications of skew lines in geometric proofs.
USEFUL FOR

Students of geometry, educators teaching geometric principles, and anyone interested in advanced geometric proofs involving three-dimensional figures.

Britney2
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2 isosceles triangles ABC and ABD have the same base AB but are in different planes.
Prove that CD is perpendicular to AB.

(It must be proved that CD is perpendicular to one of the planes and then it is perpendicular with every segment of this plane.)
Thanks in advance!
 
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Consider point O, the midpoint of AB. Construct a plane that is perpendicular to AB and contains point O. What special property does this plane have?
 


Frankly, that seems to me to use a non-standard definition of "perpendicular".

To me the standard definition of perpendicular is "lines p and q are perpendicular if and only if they meet forming right angles." For this two be true we must also allow two skew lines to be perpendicular as long as the projection of one onto a plane containing the other intersects it forming right angles.
 

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