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Pythagoras' theorem - in a right-angled triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides.

Angles in a triangle add up to 180° and in quadrilaterals add up to 360°. Angles can be calculated inside semicircles and circles. Parallel lines in shapes can form corresponding and alternate angles.

Similar figures are identical in shape, but generally not in size. A missing length on a reduction/enlargement figure can be calculated by finding its linear scale factor.