From geometry terms and definitions ,we know

An exterior angle (or external angle) is an angle formed by one side of a simple polygon and a line extended from an adjacent side.

In any triangle, if one of the sides is produced, then the exterior angle is greater than either of the interior and opposite angles,

The sum of the internal angle and the external angle on the same vertex is 180°.

The sum of the measures of the external angles of a polygon ,one at each vertex is 360°.

Here angle 1,2 ,3 are external angles

1+2+3 = 360°.

Let’s see an example on this….from 10 grade math geometry

Find the value of x in the figure….

Since we have a straight line and a part of it is 80

Angle x=180-80

So x=100°

This also help solving slope of a line problems in algebra ii

Most of the students are don’t realize what is Universal Set U
is all about ,Let’s see an example problem which explains us more ….

Question:-

Out of forty students, 14 are taking English Composition and 29 are taking Chemistry. If five students are in both classes, how many students are in neither class? How many are in either class? What is the probability that a randomly-chosen student from this group is taking only the Chemistry class?

Answer:-

This online help with math gives the solution of the above problem.
Here the total number of students in class is what is Universal Set U ,which is given as 40 students

Here is the venn diagram template for the complete question

Two students are taking neither class.
There are 38 students in at least one of the classes.
There is a 24/40 = 0.6 = 60%  probability that a randomly-chosen student in this
group is taking Chemistry but not English.