Step by step
The question
Question: if the exterior angle is 135° and the far inside angles are x° and 45°, what is x?
The exterior angle is outside the triangle, on the pushed-out side.
1 Use the rule
The exterior angle is the two far inside angles added together.
So x + 45 = 135.
Careful!
The outside angle is not inside the triangle.
It does not go in the 180° sum.
2 Remove the + 45
We want x by itself.
The + 45 is in the way.
To remove + 45, we take away 45.
We do it on both sides.
Why both sides?
The = sign means both sides are the same amount.
If we change only one side, they are not the same any more.
+ 45 and − 45 make zero.
Cross them out.
135 − 45 = 90.
So x = 90!
3 Check
Let's check: with x = 90, the inside angles are 90° and 45°.
Together 135°, the outside angle.
So the answer is right!
Let's look back.
First: use the rule.
Then: remove the + 45.
Last: check.