complete the missing parts of the paragraph proof by the definition of angles , the sum of the measure of angle 1 and 2 is 180 degrees

Well, I believe that it's letting you know  is simply just a fraction of a full number {1}

For example, You have a pie. Your Mother cut it in 7 slices, if your family ate 5 of the seven slices, there would be how many slices left?
The would be  of one whole pie.

So, I hope you find this helpful. :3