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What is the solution to the trigonometric inequality [tex]\sin^2(x) \ \textgreater \ 1 + \cos(x)[/tex] over the interval [tex]0 \leq x \leq 2\pi[/tex] radians?

A. [tex]\frac{\pi}{4} \ \textless \ x \ \textless \ \frac{7\pi}{4}[/tex]

B. [tex]\frac{\pi}{4} \ \textless \ x \ \textless \ \pi[/tex] and [tex]\pi \ \textless \ x \ \textless \ \frac{7\pi}{4}[/tex]

C. [tex]\frac{\pi}{2} \ \textless \ x \ \textless \ \frac{3\pi}{2}[/tex]

D. [tex]\frac{\pi}{2} \ \textless \ x \ \textless \ \pi[/tex] and [tex]\pi \ \textless \ x \ \textless \ \frac{3\pi}{2}[/tex]

Sagot :

To solve the trigonometric inequality \(\sin^2(x) > 1 + \cos(x)\) over the interval \(0 \leq x \leq 2\pi\), let's follow a step-by-step approach.

1. Rewriting the Inequality:

Using the Pythagorean identity for sine squared, \(\sin^2(x) = 1 - \cos^2(x)\), we can rewrite the inequality:
[tex]\[ 1 - \cos^2(x) > 1 + \cos(x) \][/tex]

2. Simplifying the Inequality:

Next, we subtract 1 from both sides:
[tex]\[ 1 - \cos^2(x) - 1 > \cos(x) \][/tex]
Simplifying further, we get:
[tex]\[ -\cos^2(x) > \cos(x) \][/tex]
Move all terms to one side to obtain:
[tex]\[ -\cos^2(x) - \cos(x) > 0 \][/tex]

3. Factoring the Expression:

We can factor out \(-\cos(x)\):
[tex]\[ \cos(x) (\cos(x) + 1) < 0 \][/tex]

4. Analyzing the Inequality:

The product \(\cos(x) (\cos(x) + 1)\) is less than 0, meaning one factor must be positive and the other negative. We analyze where \(\cos(x)\) and \(\cos(x) + 1\) have opposite signs:

- \(\cos(x)\) is negative when \( \frac{\pi}{2} < x < \frac{3\pi}{2} \).
- \(\cos(x) + 1\) is negative when \(\cos(x) < -1\), which is out of the range, so this doesn't affect our interval.

Therefore, \(\cos(x) (\cos(x) + 1)\) is negative when:
[tex]\[ \frac{\pi}{2} < x < \pi \][/tex]
and
[tex]\[ \pi < x < \frac{3\pi}{2} \][/tex]

5. Combining the Intervals:

Combining the valid intervals, we get:
[tex]\[ \frac{\pi}{2} < x < \pi \quad \text{and} \quad \pi < x < \frac{3\pi}{2} \][/tex]

The solution to the inequality \(\sin^2(x) > 1 + \cos(x)\) over the interval \(0 \leq x \leq 2\pi\) is the combined intervals:
[tex]\[ \frac{\pi}{2} < x < \pi \quad \text{and} \quad \pi < x < \frac{3\pi}{2} \][/tex]

So, the correct answer is:
[tex]\[ \boxed{\frac{\pi}{2} < x < \pi \text{ and } \pi < x < \frac{3\pi}{2}} \][/tex]