Option C:
x = 90°
Solution:
Given equation:

<u>To find the degree:</u>

Subtract 1 + cos²x from both sides.

Using the trigonometric identity:




Let sin x = u

Factor the quadratic equation.

u + 2 = 0, u – 1 = 0
u = –2, u = 1
That is sin x = –2, sin x = 1
sin x can't be smaller than –1 for real solutions. So ignore sin x = –2.
sin x = 1
The value of sin is 1 for 90°.
x = 90°.
Option C is the correct answer.
<span>The equation is not quadratic in for because it cannot be written as a second degree polynomial</span>
Suppose we choose

and

. Then

Now suppose we choose

such that

where we pick the solution for this system such that

. Then we find

Note that you can always find a solution to the system above that satisfies

as long as

. What this means is that you can always find the value of

as a (constant) function of

.
I have attached a diagram of the triangle described.
We can use any of the trigonometric functions to find angle x. Remember, SOH CAH TOA. And since we're finding the angle, we'll need to use an inverse trigonometric function. For this problem, I'll be using the sine function.
sin(x) = 84 / 85
x = sin^-1(84/85)
x = 81.2026 degrees (feel free to round to however many places you need)
Hope this helps!! :)
Coefficients are the numbers in a equation variables are the letter