Step by step
The question
Question: find what kind of triangle has angles 20°, 100°, x.
1 Find the missing angle
The three angles in a triangle add up to 180°.
So 20 + 100 + x = 180.
2 Group like terms
Terms are the parts between the + and − signs.
Like terms are the same kind: same letters, same powers.
Plain numbers go together.
20 + 100 = 120.
Now we have 120 + x = 180.
3 Remove the 120
We want x by itself.
The 120 is in the way.
To remove 120, we take away 120.
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.
120 and − 120 make zero.
Cross them out.
180 − 120 = 60.
So x = 60!
4 Find the biggest angle
The biggest angle is 100°.
Only the biggest angle decides the name.
100 is more than 90.
Careful!
Every triangle has at least two acute angles.
So they do not decide.
Only the biggest angle does.
5 Name it
So the triangle is obtuse.
6 Check
Let's check: the angles must add up to 180°.
They do.
So it is a real triangle, and it is obtuse.
Let's look back.
First: find the missing angle.
Then: group like terms.
Then: remove the 120.
Then: find the biggest angle.
Then: name it.
Last: check.