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  2. Area of a triangle - Wikipedia

    en.wikipedia.org/wiki/Area_of_a_triangle

    In geometry, calculating the area of a triangle is an elementary problem encountered often in many different situations. The best known and simplest formula is where b is the length of the base of the triangle, and h is the height or altitude of the triangle. The term "base" denotes any side, and "height" denotes the length of a perpendicular ...

  3. Viviani's theorem - Wikipedia

    en.wikipedia.org/wiki/Viviani's_theorem

    This proof depends on the readily-proved proposition that the area of a triangle is half its base times its height—that is, half the product of one side with the altitude from that side. [2] Let ABC be an equilateral triangle whose height is h and whose side is a.

  4. Equilateral triangle - Wikipedia

    en.wikipedia.org/wiki/Equilateral_triangle

    An equilateral triangle with a side of 2 has a height of √ 3, as the sine of 60° is √ 3 /2. The legs of either right triangle formed by an altitude of the equilateral triangle are half of the base , and the hypotenuse is the side of the equilateral triangle.

  5. Heron's formula - Wikipedia

    en.wikipedia.org/wiki/Heron's_formula

    Heron's formula. A triangle with sides a, b, and c. In geometry, Heron's formula (or Hero's formula) gives the area of a triangle in terms of the three side lengths ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ Letting ⁠ ⁠ be the semiperimeter of the triangle, the area ⁠ ⁠ is [1] It is named after first-century engineer Heron of Alexandria (or Hero) who ...

  6. Morley's trisector theorem - Wikipedia

    en.wikipedia.org/wiki/Morley's_trisector_theorem

    Recent proofs include an algebraic proof by Alain Connes (1998, 2004) extending the theorem to general fields other than characteristic three, and John Conway's elementary geometry proof. [2] [3] The latter starts with an equilateral triangle and shows that a triangle may be built around it which will be similar to any selected

  7. Pick's theorem - Wikipedia

    en.wikipedia.org/wiki/Pick's_theorem

    The first part of this proof shows that a triangle with three integer vertices and no other integer points has area exactly , as Pick's formula states. The proof uses the fact that all triangles tile the plane, with adjacent triangles rotated by 180° from each other around their shared edge. [9]

  8. Ptolemy's theorem - Wikipedia

    en.wikipedia.org/wiki/Ptolemy's_theorem

    Proof: It is known that the area of a triangle inscribed in a circle of radius is: = Writing the area of the quadrilateral as sum of two triangles sharing the same circumscribing circle, we obtain two relations for each decomposition.

  9. Euclidean geometry - Wikipedia

    en.wikipedia.org/wiki/Euclidean_geometry

    A proof from Euclid's Elements that, given a line segment, one may construct an equilateral triangle that includes the segment as one of its sides: an equilateral triangle ΑΒΓ is made by drawing circles Δ and Ε centered on the points Α and Β, and taking one intersection of the circles as the third vertex of the triangle.