Step by step
The question
Question: find the base angles x of an isosceles triangle with top angle 120°.
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 x too.
2 Use the angle fact
The three angles in a triangle add up to 180°: x + x + 120 = 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.
All the x terms go together.
x + x = 2x.
Now we have 2x + 120 = 180.
We want x by itself.
Two things are stuck to x: times 2 and + 120.
The + 120 is on the outside, so it comes off first.
Then the times 2.
4 Remove the + 120
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.
Now we have 2x = 60.
5 Remove the 2
2x means 2 times x.
We want just x.
To remove times 2, we divide by 2.
We do it on both sides, so they stay equal.
2 divided by 2 is 1.
So just x is left.
60 divided by 2 is 30.
So x = 30!
6 Check
Let's check: put x = 30 back and add the three angles.
They add up to 180°.
So x = 30 is right!
Let's look back.
First: use the isosceles fact.
Then: use the angle fact.
Then: group like terms.
Then: remove the + 120.
Then: remove the 2.
Last: check.