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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

Circumference and perimeter

Pi starts by measuring a circle

diameter = d
d1d2d3d+ a bitCircle perimetermeasure across: diameterrim unwraps to about 3.14 dsame ratio for every circlemany straight sides approach the rimcircumference = pi x diameterpi = C / d

Take any circle and measure straight across the middle. That distance is the diameter.