Right Angle -- from Wolfram MathWorld

A right angle is an angle equal to half the angle from one end of a line segment to the other. A right angle is pi/2 radians or 90 degrees. A triangle containing a right angle is called a right triangle. However, a triangle cannot contain more than one right angle, since the sum of the two right angles plus the third angle would exceed the 180 degrees total possessed by a triangle. The patterns of cracks observed in mud that has been dried by the sun form curves that often intersect in right

Figure no. 4. Euler's angles Source

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