Taken as separate digits.........and summed...................the digits of many of the major shapes........add up to 9.............a triangle has 180 degrees...............as does a straight line.............

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The right triangle has one 90 degree angle and two acute (< 90 degree) angles. Since the sum of the angles of a triangle is always 180 degrees... The two sides of the triangle that are by the right angle are called the legs... and the side opposite of the right angle is called the hypotenuse.

Cool math .com - Types of Triangles

www.coolmath.com/reference/triangles-types

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Triangles Contain 180 Degrees - Math is Fun

https://www.mathsisfun.com/proof180deg.html

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In a triangle, the three interior angles always add to 180°: A + B + C = 180° ... MathsIsFun.com v0.89. We can use that fact to find a missing angle in a triangle: ...

Cool math .com - Types of Triangles

www.coolmath.com/reference/triangles-types

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The right triangle has one 90 degree angle and two acute (< 90 degree) angles. Since the sum of the angles of a triangle is always 180 degrees... The two sides of the triangle that are by the right angle are called the legs... and the side opposite of the right angle is called the hypotenuse.