Trigonometry Examples

$\cot^{2} (x) - \cos^{2} (x) = \cot^{2} (x) \cos^{2} (x)$

Step 1

Start on the left side.

$\cot^{2} (x) - \cos^{2} (x)$

Step 2

Convert to sines and cosines.

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Step 2.1

Write $\cot (x)$ in sines and cosines using the quotient identity.

${(\frac{\cos (x)}{\sin (x)})}^{2} - \cos^{2} (x)$

Step 2.2

Apply the product rule to $\frac{\cos (x)}{\sin (x)}$ .

$\frac{\cos^{2} (x)}{\sin^{2} (x)} - \cos^{2} (x)$

Step 3

Write $- \cos^{2} (x)$ as a fraction with denominator $1$ .

$\frac{\cos^{2} (x)}{\sin^{2} (x)} + \frac{- \cos^{2} (x)}{1}$

Step 4

Add fractions.

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Step 4.1

To write $\frac{- \cos^{2} (x)}{1}$ as a fraction with a common denominator, multiply by $\frac{\sin^{2} (x)}{\sin^{2} (x)}$ .

$\frac{\cos^{2} (x)}{\sin^{2} (x)} + \frac{- \cos^{2} (x)}{1} \cdot \frac{\sin^{2} (x)}{\sin^{2} (x)}$

Step 4.2

Multiply $\frac{- \cos^{2} (x)}{1}$ by $\frac{\sin^{2} (x)}{\sin^{2} (x)}$ .

$\frac{\cos^{2} (x)}{\sin^{2} (x)} + \frac{- \cos^{2} (x) \sin^{2} (x)}{\sin^{2} (x)}$

Step 4.3

Combine the numerators over the common denominator.

$\frac{\cos^{2} (x) - \cos^{2} (x) \sin^{2} (x)}{\sin^{2} (x)}$

Step 5

Apply Pythagorean identity.

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Step 5.1

Multiply by $1$ .

$\frac{\cos^{2} (x) \cdot 1 - \cos^{2} (x) \sin^{2} (x)}{\sin^{2} (x)}$

Step 5.2

Factor $\cos^{2} (x)$ out of $- \cos^{2} (x) \sin^{2} (x)$ .

$\frac{\cos^{2} (x) \cdot 1 + \cos^{2} (x) (- \sin^{2} (x))}{\sin^{2} (x)}$

Step 5.3

Factor $\cos^{2} (x)$ out of $\cos^{2} (x) \cdot 1 + \cos^{2} (x) (- \sin^{2} (x))$ .

$\frac{\cos^{2} (x) \cdot (1 - \sin^{2} (x))}{\sin^{2} (x)}$

Step 5.4

Apply pythagorean identity.

$\frac{\cos^{2} (x) \cdot \cos^{2} (x)}{\sin^{2} (x)}$

Step 6

Multiply ${\cos (x)}^{2}$ by ${\cos (x)}^{2}$ by adding the exponents.

$\frac{\cos^{4} (x)}{\sin^{2} (x)}$

Step 7

Rewrite $\frac{\cos^{4} (x)}{\sin^{2} (x)}$ as $\cot^{2} (x) \cos^{2} (x)$ .

$\cot^{2} (x) \cos^{2} (x)$

Step 8

Because the two sides have been shown to be equivalent, the equation is an identity.

$\cot^{2} (x) - \cos^{2} (x) = \cot^{2} (x) \cos^{2} (x)$ is an identity