In addition to learning how to fi nd the area and perimeter of a triangle, we learned how to classify triangles according to their angles as well as the lengths of their sides. Acute Triangle Right Triangle Obtuse Triangle An acute triangle is one in which all three angles are acute. (All angles measure less than 90.) A right triangle contains one right, or 90, angle. An obtuse triangle contains one obtuse angle. (The measure of an obtuse angle is greater than 90.) Note The measures of the angles in a triangle add to 180. Classify each triangle as acute, right, or obtuse. a) A B b) Y C X Z c) S R Q Solution a) This is a right triangle because A is a right angle. (mA 90) b) This is an acute triangle because each angle is acute. (The measure of each angle is less than 90.) c) This is an obtuse triangle because R is an obtuse angle. (mR is greater than 90.) EXAMPLE 3 YOU TRY 3 Classify each triangle as acute, right, or obtuse. a) C B A b) J H c) Y K X Z www.mhhe.com/messersmith Putting It All Together 329
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