Classifying Triangles — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. If two angles in a triangle measure 60 and 70 degrees, the third angle measures 50 degrees.
The angles in any triangle sum to 180, so 180 minus 60 plus 70 equals 50 degrees.
2. If two angles in a triangle measure 45 and 45 degrees, the third angle measures 90 degrees.
Adding 45 plus 45 gives 90, and 180 minus 90 leaves a 90 degree right angle.
3. If two angles in a triangle measure 30 and 60 degrees, the third angle measures 90 degrees.
180 minus 30 minus 60 equals 90 degrees, making this a right triangle.
4. A triangle has angles of 80 and 70 degrees. Its third angle measures 30 degrees.
80 plus 70 equals 150, and 180 minus 150 leaves 30 degrees for the third angle.
5. If a triangle has angles of 100 and 40 degrees, the third angle measures 40 degrees.
100 plus 40 equals 140, and 180 minus 140 equals 40 degrees, making the triangle isosceles.
6. A triangle has two angles of 25 and 65 degrees. The third angle measures 90 degrees.
Since 25 plus 65 equals 90, the third angle is 180 minus 90, which is 90 degrees.
7. If a triangle has angles 110 and 35 degrees, the third angle measures 35 degrees.
110 plus 35 equals 145, and 180 minus 145 equals 35 degrees, so the triangle is also isosceles.
8. A triangle has two angles measuring 60 degrees each. The third angle measures 60 degrees.
60 plus 60 equals 120, and 180 minus 120 leaves 60, making this an equilateral triangle.
9. A triangle has angles of 20 and 70 degrees. The third angle measures 90 degrees.
20 plus 70 equals 90, and 180 minus 90 equals 90 degrees, so this is a right triangle.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
Two angles 50 and 60
→ Third angle 70 degrees
Third angle 70 degrees
Two angles 45 and 45
→ Third angle 90 degrees
Third angle 90 degrees
Two angles 100 and 30
→ Third angle 50 degrees
Third angle 50 degrees
Two angles 80 and 60
→ Third angle 40 degrees
Third angle 40 degrees
The three angles of any triangle always sum to 180 degrees, so the missing angle equals 180 minus the other two.