To determine which new angle is created by extending one side of a triangle, let's carefully consider the definitions of the various angles provided as options:
1. Remote interior angle: This is an interior angle of the triangle that is not adjacent to the exterior angle. It is inside the triangle and not directly related to the extension of a side.
2. Adjacent interior angle: This is an interior angle of the triangle that is adjacent (next to) the exterior angle formed when one side of the triangle is extended. It is still inside the triangle.
3. Exterior angle: When you extend one side of a triangle, you create an angle outside the triangle between the extended side and the adjacent side of the triangle. This is known as the exterior angle.
4. Complementary angle: This is an angle that, when added to another angle, sums to 90 degrees. This concept generally applies to pairs of angles and is not specific to triangle side extensions.
Given these definitions, when you extend one side of a triangle, the new angle formed outside the triangle is called an exterior angle. This angle is formed between the extended side of the triangle and the adjacent side, lying outside the boundaries of the original triangle. Therefore, the correct answer is:
An exterior angle.
