The sum of the three included angles of a triangle always = 180 degrees.
So if one angle = 180 degrees (a straight line) then the other two included angles will be = 0 degrees which cannot make a triangle.
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Because the 3 interior angles of a triangle must add up to 180 degrees and not 1 angle on its own
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If the triangle has a 30 degree angle and a 60 degree angle then the 3rd angle must be a 90 degree angle because there are 180 degrees in a triangle and so therefore it is a right angle triangle.
By definition, a triangle must have three angles. It is also known that the interior angles of a triangle must total 180 degrees. If a triangle contained a single interior angle of 180 degrees, the other angles of the supposed triangle would both have to be zero, and the supposed triangle would be a straight line.
If a triangle has a 100 degree angle, it is an obtuse triangle because a 100 degree angle is an obtuse angle. angle > 90 = obtuse angle = 90 = right angle < 90 = acute