Answer:
12
Step-by-step explanation:
AC = BD (as the diagonal of the rectangle)
4x-60=30-x
4x+x = 30+60
5x = 90
x = 90/5
x = 18
=> BD = 30-18 = 12
The expression that is not a variation of the Pythagorean identity is the third option.
<h3>
What is the Pythagorean identity?</h3>
The Pythagorean identity can be written as:

For example, if we subtract cos^2(x) on both sides we get the second option:

Which is a variation.
Now, let's divide both sides by cos^2(x).

Notice that the third expression in the options looks like this one, but the one on the right side is positive. The above expression is in did a variation of the Pythagorean identity, then the one written in the options (with the 1 instead of the -1) is incorrect, meaning that it is not a variation of the Pythagorean identity.
Concluding, the correct option is the third one.
If you want to learn more about the Pythagorean identity, you can read:
brainly.com/question/24287773
Answer:
<em>(c). </em>
<em> </em>
Step-by-step explanation:
The <u><em>cosine</em></u> is the <em>"x" coordinate</em> of the terminal point on the unit circle.
Thus the answer is <em>(c). </em>