What is an obtuse angle?
(an angle whose measure is greater than 90 degrees and less than 180 degrees) • Say: Good. I’m drawing a triangle with an obtuse angle. (Draw an obtuse triangle on the board.) This is called an obtuse triangle because it has an obtuse angle. • Say: If a triangle has only acute angles, we call it an acute triangle. Thus, we have three ways to classify triangles based on angle measures-acute, right, and obtuse. Write these three names on the board next to the name for classifying triangles by the lengths of their sides. Now, draw a picture of any quadrilateral on the board. • Ask: What do we call a polygon with four sides? (a quadrilateral) We can draw a line from one vertex to an opposite vertex like this. (Draw this on the board.) This creates two triangles. What is the sum of the angle measures for each triangle? (180 degrees) That’s right. Therefore, the sum of the angle measures for a quadrilateral is 360 degrees. On the board, tape the picture of the trapezoid. • Ask: Does anyone k
