Complementary Angles

Two positive angles are complementary if their angle measures sum up to 90^\circ . When they are adjacent to each other, you’ll realize that they form a right angle. A right angle measures exactly 90^\circ , so are complementary angles.

In our illustration above, \angle QXR and \angle RXS are complementary. Why? Because these two angles complement each other with their angle measures adding up to 90^\circ .

However, two angles don’t need to be adjacent or have a common side and vertex to be called complementary angles. If their angle measures add up to 90^\circ , they are also complementary.

Above we have two non-adjacent angles, \angle K and \angle N. But since the m\angle K = 48^\circ and m\angle N = 42^\circ add up to 90^\circ , they are also called complementary angles.

Tip: Be careful to spell it as “complement”, with an “e”, and NOT as “compliment”. However, I’m sure you’ll be getting a lot of compliments if you spell it correctly each time. 🙂


Example Problems Involving Complementary Angles

The best way to get familiar with the relationship of this angle pair is by going through various examples.

Example 1: What is the measure of \angle EHF?

Here we have two complementary angles that are next to each other forming the right angle, \angle EHG. However, we are only given the angle measure of \angle FHG which is 27^\circ .

Since we know that right angles measure exactly 90^\circ , then it’s safe to say that the measures of \angle EHF and \angle FHG sum up to 90^\circ . We’ll use this concept to set up our equation and use the variable x in place of the missing angle measure.

So we have x = 63 which means that the measure of \angle EHF is \textbf{63}^\circ.

To check if our answer is correct, all we have to do is plug in the value of x into our original equation. If both sides of the equation equal to 90, then we got the correct measure for \angle EHF.


Example 2: Are \angle LMP and \angle UVW complementary?

At the beginning of this lesson, we discussed that angles don’t have to share the same side or vertex (adjacent) in order to be called complementary angles. If their measures sum up to 90^\circ , they are still considered as complementary.

So all we have to do in this example is add the measures of both angles to find out if the total comes up to 90^\circ .

Unfortunately, the measures of our angles did not add up to 90^\circ . Therefore, \angle LMP and \angle UVW are NOT complementary angles.


Example 3: Find the missing angle measure.

This problem is similar to our first example. We are again given a right angle formed by two adjacent and complementary angles, namely, \angle AYB and \angle BYC. Always remember that a right angle measures 90^\circ .

This time, instead of setting up an equation, let’s simply subtract the known measure (18^\circ ) from 90^\circ to find the missing measure for \angle AYB.

Is this correct? Well, the only way to find out is by checking if 18^\circ and 72^\circ add up to 90^\circ .

And they do! So, the missing angle measure or the m\angle AYB is \textbf{72}^\circ .


Example 4: If \angle J and \angle T are complementary and \angle J is 33^\circ , what is the measure of \angle T?

In this example, we are given two separate angles but their measures should total to 90^\circ because they are complementary. Since we already have the measure for \angle J, we’ll proceed by merely subtracting 33^\circ from 90^\circ . Their difference would be the angle measure for \angle T.

We get \textbf{57}^\circ as the measure for \angle T.

Let’s now add 57^\circ and 33^\circ to check if their measures sum up to 90^\circ .


Example 5: Find the value of x.

The diagram gives us two angle measures with one of the measures expressed in an algebraic expression. It may look challenging but it’s actually not. As long as you know the concept of the relationship between complementary angles, you’ll be fine.

By now we know that the sum of the measures will be 90^\circ . So we’ll set up our equation by adding both measures on one side of the equation and 90^\circ on the other.

The value of x is \textbf{12}.

However, since we already know what x is, we might as well find out what the measure of \angle GRW is since it is expressed in an algebraic expression.

Awesome! The measure of \angle GRW is also 45^\circ !

Is that really correct? Well, simply add 45^\circ and 45^\circ . Both measures should total to 90^\circ .


Example 6: What is the value of x?

This next example is interesting because now both angle measures are expressed in algebraic expressions. But as I said before, just always remember that the sum of their measures will be 90^\circ since they are complementary angles. With that in mind, it’ll be easy for you to set up our equation and solve for x.

The value of x is \textbf{8}.

Let’s use this value to find out what our angle measures are.

There you have it. We have 75^\circ and 15^\circ as our angle measures. I’ll now leave it up to you to check if these measures actually sum up to 90^\circ .


Example 7: Suppose \angle AKL and \angle QBH are complementary. Find the measures of the two angles.

Note that even though they are two separate angles, they are complementary. Hence, their angle measures should sum up to 90^\circ .

We’ll now find what these measures are by first solving for x.

So we have x = 18. Let’s continue finding the measures for \angle AKL and \angle QBH.

If we add both angle measures, \textbf{66}^\circ + \textbf{24}^\circ = 90^\circ, indeed they are complementary angles.


Example 8: Determine the measures of the two complementary angles.

We have another interesting example here. Instead of algebraic expressions, our angle measures are now expressed as fractions. Let’s apply the same strategies that we used in our previous examples.

To find the m\angle \,E and m\angle \,W, we’ll simply substitute x with 216.

Both measures sum up to 90^\circ which confirms that both angles are complementary.


Example 9: Two complementary angles are such that one of the angles is twice the sum of the other angle. What are the measures of the complementary angles?

Begin by solving for x.

This time, I’ll let you find the measures for \angle VNX and \angle SKO. If you solved them correctly, below are the measures that you should get.

m\angle VNX = 30^\circ

m\angle SKO = 60^\circ


Example 10: \angle MJC is a right angle. If the measure of \angle MJT is 4 + 3x^\circ and the measure of \angle TJC is 5x + 14^\circ, what is the measure of \angle TJC?

Here’s another practice for you! Try solving this on your own first. Then check your answers by simply clicking the colored bar below.

Click here for m\angle TJC

Value of x= 9

m\angle MJT = 31^\circ

m\angle TJC =\textbf{59}^\circ