Right Obtuse And Acute Triangles. Skip to content ixl learning learning (obviously, only a single angle in a triangle can be obtuse or it wouldn’t be a triangle.) a triangle must be either obtuse, acute, or right.

If we add up the three angles of any triangle, they always equal 180 °. In respect to this, how do you know if an obtuse is acute or right? For the obtuse, right angle and acute triangles cosine of the angle opposite to the longest sides will be negative, 0 and positive, respectively.