never.
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A circle, an ellipse, a triange, any regular polygon with an odd number of sides, a general (ie not a special case) quadrilateral, are some examples
three
No. A triangle can never have parallel lines. Parallel lines run alongside each other, so that if they continued forever, they would never meet, such as the top and bottom of a square. All lines meet in a triangle. A right triangle is distinguished by having one 90 degree angle (also known as a right angle).
It is its hypotenuse
Pyramid