Concept

Isosceles triangle

Overview

triangle with two sides congruent and two angles equal

Overview adapted from Wikipedia (CC BY-SA 4.0; see the article history for authors).

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So much for their passage into one another: I must now speak of their construction. From the triangle of which the hypotenuse is twice the lesser side the three first regular solids are formed—first, the equilateral pyramid or tetrahedron; secondly, the octahedron; thirdly, the icosahedron; and from the isosceles triangle is formed the cube. And there is a fifth figure (which is made out of twelve pentagons), the…
Plato, Timaeus
States what Isosceles triangle is; names Isosceles triangle twice.
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xii. To learn the properties of the isosceles triangle the mathematical student must 'produce (by 'construction') what he himself thought into it and exhibited _à priori_ according to concepts.' 'Construction, in general,' says Schelling (_Werke,_ v. 252: cf. iv. 407) 'is the exhibition of the universal and particular in unity':--'absolute unity of the ideal and the real.' v. 225.
Georg Wilhelm Friedrich Hegel, The Logic of Hegel
States what Isosceles triangle is; names Isosceles triangle.
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Our example shall be the fifth proposition of the first book of Euclid. The inquiry is, Are the angles at the base of an isosceles triangle equal or unequal? The first thing to be considered is, what inductions we have, from which we can infer equality or inequality.
John Stuart Mill, A System of Logic, Ratiocinative and Inductive
Names Isosceles triangle.
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