Angle ABC is a straight angle. Angles ABD and DBC are right angles. Angles CBE and DBE are acute angles. Angle ABE is an obtuse angle. Angles  The sum of the three angles in any triangle is always 180°.  a + b + c = 180° Knowing that the sum of the angles of triangle is 180° can help you to solve problems about triangles. Angles  In this triangle, angle b is a right angle and angle c = 40°. What is the measure of angle a?  a + b + c = 180° Since b is a right angle, b= 90°.The problem states that c= 40°.Therefore, a + 90° + 40°= 180°a + 130°= 180°a + 130° − 130°= 180° − 130°a= 50°Angle a measures 50°. Angles Click on the figure that matches this description. An obtuse angle      Question 1 of 8 This is an acute angle. It measures less than 90°. Try again This is a right angle. The square in the corner shows that it measures 90°. Try again This is a straight angle. It measures 180°. Try again. This angle is obtuse. It is greater than 90° and less than 180°. One of these figures is an obtuse angle. Try again. An obtuse angle is larger then a right angle and smaller then a straight angle.  Angles Click on the figure that matches this description. An acute angle      Question 2 of 8 This is an acute angle. It measures less than 90°. This is a right angle. The square in the corner shows that it measures 90°. Try again This is a straight angle. It measures 180°. Try again. This angle is obtuse. It is greater than 90° and less than 180°. Try again. One of these figures is an acute angle. Try again. An acute angle is smaller then a right angle.  Angles Click on the figure that matches this description. A right angle      Question 3 of 8 This is an acute angle. It measures less than 90°. Try again This is a straight angle. It measures 180°. Try again. This angle is an obtuse angle. It measures more than 90° and less than 180°. Try again. This is an acute angle. It measures less than 90°. Try again. A right angle looks like a square corner and measures 90°. A right angle measures 90°.  Angles There are five question remaining. In each of these question, you will be asked to find the size of a missing angle. Angles  Click on the correct answer. Angle ABC is a right angle. If angle n is 40°, how large is angle m?   50° 140°   90°   60° None of these Question 4 of 8 90° − 40° = 50° Angle m is 50°. Angle ABC is 90° , and angle m is only part of this right angle. It is less than 90°. Try again. Angle ABC is 90°, and angle m is only part of this right angle. Try again. Angle m and n together from a 90° angle. Subtract 40° from 90° .Try again. One of these is size of angle m. Try again. Since angle ABC is a straight angle, angle m + angle n = 90°. ​