Step by step
The question
Question: find the supplement of 45°.
We call it x.
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: 45 + x = 180.
Careful!
Supplementary means 180, not 90.
90 is for complementary angles.
2 Remove the 45
We want x by itself.
The 45 is in the way.
To remove 45, we take away 45.
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.
45 and − 45 make zero.
Cross them out.
180 minus 45 is 135.
So x = 135.
The supplement of 45° is 135°.
3 Check
Let's check: put x = 135 back and add the angles.
They add up to 180°.
So x = 135 is right!
Let's look back.
First: use the angle fact.
Then: remove the 45.
Last: check.