Step by step
The question
Question: find x when the angles 25° and x are complementary.
1 Use the angle fact
Complementary angles add up to 90°.
Together they make a right angle: a corner.
To remember: C comes before S in the alphabet.
90 comes before 180.
So we add the two angles and make it 90: 25 + x = 90.
Careful!
Complementary means 90, not 180.
180 is for supplementary angles.
2 Remove the 25
We want x by itself.
The 25 is in the way.
To remove 25, we take away 25.
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.
25 and − 25 make zero.
Cross them out.
90 minus 25 is 65.
So x = 65.
The angle x is 65°.
3 Check
Let's check: put x = 65 back and add the angles.
They add up to 90°.
So x = 65 is right!
Let's look back.
First: use the angle fact.
Then: remove the 25.
Last: check.