Step by step
The question
Question: if the exterior angle is 100° and the far inside angles are x° and 35°, 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 + 35 = 100.
Careful!
The outside angle is not inside the triangle.
It does not go in the 180° sum.
2 Remove the + 35
We want x by itself.
The + 35 is in the way.
To remove + 35, we take away 35.
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.
+ 35 and − 35 make zero.
Cross them out.
100 − 35 = 65.
So x = 65!
3 Check
Let's check: with x = 65, the inside angles are 65° and 35°.
Together 100°, the outside angle.
So the answer is right!
Let's look back.
First: use the rule.
Then: remove the + 35.
Last: check.