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numbers and algebraic expressions
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function and its properties
linear function
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polynomials
rational function
exponential function
logarithm
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trigonometry
plane geometry
analytic geometry
solid geometry
combinatorics
probability
elements of statistics
kropki game

a + c = b + d. If sums of its opposite sides are equal then quadrilateral can be circumscribed about a circle. If quadrilateral can be circumscribed about a circle then sums of its opposite sides are equal. The radius of a circle inscribed in a quadrilateral. r = 2P / a+b+c+d. Area of a quadrilateral.