Step by step
The question
Question: find the top angle x of an isosceles triangle with a base angle of 30°.
1 Use the isosceles fact
Isosceles means two sides are equal.
The small marks show which two.
Fold it on the dashed line, from the top to the middle of the base.
The two halves fit exactly.
So the two angles at the base fit on each other.
The base angles are equal.
So the other base angle is 30° too.
2 Use the angle fact
The three angles in a triangle add up to 180°: 30 + 30 + x = 180.
Careful!
There are two base angles.
Both go in the sum.
3 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.
30 + 30 = 60.
Now we have 60 + x = 180.
4 Remove the 60
We want x by itself.
The 60 is in the way.
To remove 60, we take away 60.
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.
60 and − 60 make zero.
Cross them out.
180 − 60 = 120.
So x = 120!
5 Check
Let's check: put x = 120 back and add the three angles.
They add up to 180°.
So x = 120 is right!
Let's look back.
First: use the isosceles fact.
Then: use the angle fact.
Then: group like terms.
Then: remove the 60.
Last: check.