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Concept

Right Triangle

A right triangle is a triangle that has one right angle. The side opposite the right angle is always the longest and is known as the hypotenuse. The other sides are commonly called legs. Notice that in a right triangle, the legs are perpendicular to each other.
Right triangle vs. Not a right triangle
If one of the acute angles in the triangle is labeled, the legs can be discussed relative to that angle. Consider an acute angle labeled as on the diagram. The side that forms the angle is the adjacent side and the side not touching the angle is the opposite side.
Sides of a right triangle labeled
Note that the opposite and adjacent sides change when changes, but the hypotenuse is always the same.