Trigonometry Examples

$f (x) = 5 \cos (x)$

Step 1

Write $f (x) = 5 \cos (x)$ as an equation.

$y = 5 \cos (x)$

Step 2

Interchange the variables.

$x = 5 \cos (y)$

Step 3

Solve for $y$ .

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Step 3.1

Rewrite the equation as $5 \cos (y) = x$ .

$5 \cos (y) = x$

Step 3.2

Divide each term in $5 \cos (y) = x$ by $5$ and simplify.

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Step 3.2.1

Divide each term in $5 \cos (y) = x$ by $5$ .

$\frac{5 \cos (y)}{5} = \frac{x}{5}$

Step 3.2.2

Simplify the left side.

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Step 3.2.2.1

Cancel the common factor of $5$ .

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Step 3.2.2.1.1

Cancel the common factor.

$\frac{5 \cos (y)}{5} = \frac{x}{5}$

Step 3.2.2.1.2

Divide $\cos (y)$ by $1$ .

$\cos (y) = \frac{x}{5}$

Step 3.3

Take the inverse cosine of both sides of the equation to extract $y$ from inside the cosine.

$y = \arccos (\frac{x}{5})$

Step 4

Replace $y$ with $f^{- 1} (x)$ to show the final answer.

$f^{- 1} (x) = \arccos (\frac{x}{5})$

Step 5

Verify if $f^{- 1} (x) = \arccos (\frac{x}{5})$ is the inverse of $f (x) = 5 \cos (x)$ .

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Step 5.1

To verify the inverse, check if $f^{- 1} (f (x)) = x$ and $f (f^{- 1} (x)) = x$ .

Step 5.2

Evaluate $f^{- 1} (f (x))$ .

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Step 5.2.1

Set up the composite result function.

$f^{- 1} (f (x))$

Step 5.2.2

Evaluate $f^{- 1} (5 \cos (x))$ by substituting in the value of $f$ into $f^{- 1}$ .

$f^{- 1} (5 \cos (x)) = \arccos (\frac{5 \cos (x)}{5})$

Step 5.2.3

Cancel the common factor of $5$ .

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Step 5.2.3.1

Cancel the common factor.

$f^{- 1} (5 \cos (x)) = \arccos (\frac{5 \cos (x)}{5})$

Step 5.2.3.2

Divide $\cos (x)$ by $1$ .

$f^{- 1} (5 \cos (x)) = \arccos (\cos (x))$

Step 5.3

Evaluate $f (f^{- 1} (x))$ .

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Step 5.3.1

Set up the composite result function.

$f (f^{- 1} (x))$

Step 5.3.2

Evaluate $f (\arccos (\frac{x}{5}))$ by substituting in the value of $f^{- 1}$ into $f$ .

$f (\arccos (\frac{x}{5})) = 5 \cos (\arccos (\frac{x}{5}))$

Step 5.3.3

The functions cosine and arccosine are inverses.

$f (\arccos (\frac{x}{5})) = 5 (\frac{x}{5})$

Step 5.3.4

Cancel the common factor of $5$ .

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Step 5.3.4.1

Cancel the common factor.

$f (\arccos (\frac{x}{5})) = 5 (\frac{x}{5})$

Step 5.3.4.2

Rewrite the expression.

$f (\arccos (\frac{x}{5})) = x$

Step 5.4

Since $f^{- 1} (f (x)) = x$ and $f (f^{- 1} (x)) = x$ , then $f^{- 1} (x) = \arccos (\frac{x}{5})$ is the inverse of $f (x) = 5 \cos (x)$ .

$f^{- 1} (x) = \arccos (\frac{x}{5})$