1. Angle 1 and angle three are congruent--this is given. 2. Angle R and angle V are congruent--this is also given. 3. If two sets of angles in two different triangles are congruent, then the sides opposite these angles are congruent. 4. Thus, US and UT are congruent. 5. By the definition of an isosceles triangle, this means that STU is isosceles.