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Algebra
Balancing EquationsSimultaneous EquationsDistributive PropertyFactoring QuadraticsCommon FactorDifference of SquaresFactor by GroupingCompleting the SquareQuadratic FormulasCubic FormulasPower Formulas
Geometry
Area of a Triangle
Trigonometry
SAS Triangle AreaSin, Cos, TanHow Sine Is Derived
Circles
How Pi Is DerivedArea of a CircleArea of a SectorPerimeter of a SectorRadians

Animated proof

Start with one triangle

A = 1/2 x b x h
two triangles = base x heightbaseheight1 triangle = half

The base is the bottom edge. The height is the straight-up distance from the base to the opposite corner.