Step by step
The question
Question: if the exterior angle is 120° and the far inside angles are 70° and x°, 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 70 + x = 120.
Careful!
The outside angle is not inside the triangle.
It does not go in the 180° sum.
2 Remove the 70
We want x by itself.
The 70 is in the way.
To remove 70, we take away 70.
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.
70 and − 70 make zero.
Cross them out.
120 − 70 = 50.
So x = 50!
3 Check
Let's check: with x = 50, the inside angles are 70° and 50°.
Together 120°, the outside angle.
So the answer is right!
Let's look back.
First: use the rule.
Then: remove the 70.
Last: check.