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This essential trains for: SAT-I, GMAT, Math Kangaroo 5-6, Math Kangaroo 7-8.

Triangles are polygons with three angles (and three sides).

Sum of angles: The sum of the angles of a triangle is 180°.

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Consequence: Only one angle of a triangle may be obtuse.

Triangle inequality: For three segments to form a triangle, any two of them must have a combined length that exceeds the length of the third one.

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Isosceles triangle:

The line of symmetry of an isosceles triangle is also:

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Equilateral triangle:

The lines of symmetry of an equilateral triangle are also:

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Note: The plane may be tiled with equilateral triangles.

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A right-angle triangle: is half of a rectangle.

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