To find the reference angle for $\cos(225^\circ)$, follow these steps:

1. First, note that $225^\circ$ is in the third quadrant, where cosine is negative.
2. The reference angle is the smallest angle formed between the terminal side of the angle and the x-axis.
3. In the third quadrant, the reference angle is found by subtracting $180^\circ$ from the given angle: $\text{Reference angle} = 225^\circ - 180^\circ = 45^\circ$

Thus, the reference angle for $225^\circ$ is $45^\circ$.

Would you like to go over further details or have any questions?

Here are 5 related questions:

1. What is the sine of $225^\circ$?
2. How do you determine the reference angle for an angle in the fourth quadrant?
3. What is the cosine of a reference angle of $30^\circ$?
4. How does cosine behave in the third quadrant?
5. What is the general formula for finding a reference angle in any quadrant?

Tip: The reference angle is always between $0^\circ$ and $90^\circ$, regardless of which quadrant the original angle lies in.