Step by step
The question
Question: find x when the angles x and 72° are supplementary.
1 Use the angle fact
Supplementary angles add up to 180°.
Together they make a straight line.
To remember: C comes before S in the alphabet.
90 comes before 180.
So we add the two angles and make it 180: x + 72 = 180.
Careful!
Supplementary means 180, not 90.
90 is for complementary angles.
2 Remove the + 72
We want x by itself.
The + 72 is in the way.
To remove + 72, we take away 72.
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.
+ 72 and − 72 make zero.
Cross them out.
180 minus 72 is 108.
So x = 108.
The angle x is 108°.
3 Check
Let's check: put x = 108 back and add the angles.
They add up to 180°.
So x = 108 is right!
Let's look back.
First: use the angle fact.
Then: remove the + 72.
Last: check.