Step by step
The question
Question: find x when the angles in a triangle are x, x and 40°.
1 Use the angle fact
The three angles in a triangle always add up to 180°.
So we add them all and make it 180: x + x + 40 = 180.
2 Group like terms
The x terms go together.
Now we have 2x + 40 = 180.
3 Remove the + 40
We want 2x by itself.
The + 40 is in the way.
To remove + 40, we take away 40.
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.
+ 40 and − 40 make zero.
Cross them out.
180 minus 40 is 140.
4 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.
140 divided by 2 is 70.
So x = 70.
The angle x is 70°.
5 Check
Let's check: put x = 70 back and add the angles.
They add up to 180°.
So x = 70 is right!
Let's look back.
First: use the angle fact.
Then: group like terms.
Then: remove the + 40.
Then: remove the 2.
Last: check.