Calculus Examples

$f (x) = \sqrt{x^{2} + 9}$ , $x = 4$

Step 1

Find the corresponding $y$ -value to $x = 4$ .

Tap for more steps...

Step 1.1

Substitute $4$ in for $x$ .

$y = \sqrt{{(4)}^{2} + 9}$

Step 1.2

Simplify $\sqrt{{(4)}^{2} + 9}$ .

Tap for more steps...

Step 1.2.1

Raise $4$ to the power of $2$ .

$y = \sqrt{16 + 9}$

Step 1.2.2

Add $16$ and $9$ .

$y = \sqrt{25}$

Step 1.2.3

Rewrite $25$ as $5^{2}$ .

$y = \sqrt{5^{2}}$

Step 1.2.4

Pull terms out from under the radical, assuming positive real numbers.

$y = 5$

Step 2

Find the first derivative and evaluate at $x = 4$ and $y = 5$ to find the slope of the tangent line.

Tap for more steps...

Step 2.1

Use $\sqrt[n]{a^{x}} = a^{\frac{x}{n}}$ to rewrite $\sqrt{x^{2} + 9}$ as ${(x^{2} + 9)}^{\frac{1}{2}}$ .

$\frac{d}{d x} [{(x^{2} + 9)}^{\frac{1}{2}}]$

Step 2.2

Differentiate using the chain rule, which states that $\frac{d}{d x} [f (g (x))]$ is $f' (g (x)) g' (x)$ where $f (x) = x^{\frac{1}{2}}$ and $g (x) = x^{2} + 9$ .

Tap for more steps...

Step 2.2.1

To apply the Chain Rule, set $u$ as $x^{2} + 9$ .

$\frac{d}{d u} [u^{\frac{1}{2}}] \frac{d}{d x} [x^{2} + 9]$

Step 2.2.2

Differentiate using the Power Rule which states that $\frac{d}{d u} [u^{n}]$ is $n u^{n - 1}$ where $n = \frac{1}{2}$ .

$\frac{1}{2} u^{\frac{1}{2} - 1} \frac{d}{d x} [x^{2} + 9]$

Step 2.2.3

Replace all occurrences of $u$ with $x^{2} + 9$ .

$\frac{1}{2} {(x^{2} + 9)}^{\frac{1}{2} - 1} \frac{d}{d x} [x^{2} + 9]$

Step 2.3

To write $- 1$ as a fraction with a common denominator, multiply by $\frac{2}{2}$ .

$\frac{1}{2} {(x^{2} + 9)}^{\frac{1}{2} - 1 \cdot \frac{2}{2}} \frac{d}{d x} [x^{2} + 9]$

Step 2.4

Combine $- 1$ and $\frac{2}{2}$ .

$\frac{1}{2} {(x^{2} + 9)}^{\frac{1}{2} + \frac{- 1 \cdot 2}{2}} \frac{d}{d x} [x^{2} + 9]$

Step 2.5

Combine the numerators over the common denominator.

$\frac{1}{2} {(x^{2} + 9)}^{\frac{1 - 1 \cdot 2}{2}} \frac{d}{d x} [x^{2} + 9]$

Step 2.6

Simplify the numerator.

Tap for more steps...

Step 2.6.1

Multiply $- 1$ by $2$ .

$\frac{1}{2} {(x^{2} + 9)}^{\frac{1 - 2}{2}} \frac{d}{d x} [x^{2} + 9]$

Step 2.6.2

Subtract $2$ from $1$ .

$\frac{1}{2} {(x^{2} + 9)}^{\frac{- 1}{2}} \frac{d}{d x} [x^{2} + 9]$

Step 2.7

Combine fractions.

Tap for more steps...

Step 2.7.1

Move the negative in front of the fraction.

$\frac{1}{2} {(x^{2} + 9)}^{- \frac{1}{2}} \frac{d}{d x} [x^{2} + 9]$

Step 2.7.2

Combine $\frac{1}{2}$ and ${(x^{2} + 9)}^{- \frac{1}{2}}$ .

$\frac{{(x^{2} + 9)}^{- \frac{1}{2}}}{2} \frac{d}{d x} [x^{2} + 9]$

Step 2.7.3

Move ${(x^{2} + 9)}^{- \frac{1}{2}}$ to the denominator using the negative exponent rule $b^{- n} = \frac{1}{b^{n}}$ .

$\frac{1}{2 {(x^{2} + 9)}^{\frac{1}{2}}} \frac{d}{d x} [x^{2} + 9]$

Step 2.8

By the Sum Rule, the derivative of $x^{2} + 9$ with respect to $x$ is $\frac{d}{d x} [x^{2}] + \frac{d}{d x} [9]$ .

$\frac{1}{2 {(x^{2} + 9)}^{\frac{1}{2}}} (\frac{d}{d x} [x^{2}] + \frac{d}{d x} [9])$

Step 2.9

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 2$ .

$\frac{1}{2 {(x^{2} + 9)}^{\frac{1}{2}}} (2 x + \frac{d}{d x} [9])$

Step 2.10

Since $9$ is constant with respect to $x$ , the derivative of $9$ with respect to $x$ is $0$ .

$\frac{1}{2 {(x^{2} + 9)}^{\frac{1}{2}}} (2 x + 0)$

Step 2.11

Simplify terms.

Tap for more steps...

Step 2.11.1

Add $2 x$ and $0$ .

$\frac{1}{2 {(x^{2} + 9)}^{\frac{1}{2}}} (2 x)$

Step 2.11.2

Combine $2$ and $\frac{1}{2 {(x^{2} + 9)}^{\frac{1}{2}}}$ .

$\frac{2}{2 {(x^{2} + 9)}^{\frac{1}{2}}} x$

Step 2.11.3

Combine $\frac{2}{2 {(x^{2} + 9)}^{\frac{1}{2}}}$ and $x$ .

$\frac{2 x}{2 {(x^{2} + 9)}^{\frac{1}{2}}}$

Step 2.11.4

Cancel the common factor.

$\frac{2 x}{2 {(x^{2} + 9)}^{\frac{1}{2}}}$

Step 2.11.5

Rewrite the expression.

$\frac{x}{{(x^{2} + 9)}^{\frac{1}{2}}}$

Step 2.12

Evaluate the derivative at $x = 4$ .

$\frac{4}{{({(4)}^{2} + 9)}^{\frac{1}{2}}}$

Step 2.13

Simplify the denominator.

Tap for more steps...

Step 2.13.1

Raise $4$ to the power of $2$ .

$\frac{4}{{(16 + 9)}^{\frac{1}{2}}}$

Step 2.13.2

Add $16$ and $9$ .

$\frac{4}{25^{\frac{1}{2}}}$

Step 2.13.3

Rewrite $25$ as $5^{2}$ .

$\frac{4}{{(5^{2})}^{\frac{1}{2}}}$

Step 2.13.4

Apply the power rule and multiply exponents, ${(a^{m})}^{n} = a^{m n}$ .

$\frac{4}{5^{2 (\frac{1}{2})}}$

Step 2.13.5

Cancel the common factor of $2$ .

Tap for more steps...

Step 2.13.5.1

Cancel the common factor.

$\frac{4}{5^{2 (\frac{1}{2})}}$

Step 2.13.5.2

Rewrite the expression.

$\frac{4}{5^{1}}$

Step 2.13.6

Evaluate the exponent.

$\frac{4}{5}$

Step 3

Plug the slope and point values into the point-slope formula and solve for $y$ .

Tap for more steps...

Step 3.1

Use the slope $\frac{4}{5}$ and a given point $(4, 5)$ to substitute for $x_{1}$ and $y_{1}$ in the point-slope form $y - y_{1} = m (x - x_{1})$ , which is derived from the slope equation $m = \frac{y_{2} - y_{1}}{x_{2} - x_{1}}$ .

$y - (5) = \frac{4}{5} \cdot (x - (4))$

Step 3.2

Simplify the equation and keep it in point-slope form.

$y - 5 = \frac{4}{5} \cdot (x - 4)$

Step 3.3

Solve for $y$ .

Tap for more steps...

Step 3.3.1

Simplify $\frac{4}{5} \cdot (x - 4)$ .

Tap for more steps...

Step 3.3.1.1

Rewrite.

$y - 5 = 0 + 0 + \frac{4}{5} \cdot (x - 4)$

Step 3.3.1.2

Simplify by adding zeros.

$y - 5 = \frac{4}{5} \cdot (x - 4)$

Step 3.3.1.3

Apply the distributive property.

$y - 5 = \frac{4}{5} x + \frac{4}{5} \cdot - 4$

Step 3.3.1.4

Combine $\frac{4}{5}$ and $x$ .

$y - 5 = \frac{4 x}{5} + \frac{4}{5} \cdot - 4$

Step 3.3.1.5

Multiply $\frac{4}{5} \cdot - 4$ .

Tap for more steps...

Step 3.3.1.5.1

Combine $\frac{4}{5}$ and $- 4$ .

$y - 5 = \frac{4 x}{5} + \frac{4 \cdot - 4}{5}$

Step 3.3.1.5.2

Multiply $4$ by $- 4$ .

$y - 5 = \frac{4 x}{5} + \frac{- 16}{5}$

Step 3.3.1.6

Move the negative in front of the fraction.

$y - 5 = \frac{4 x}{5} - \frac{16}{5}$

Step 3.3.2

Move all terms not containing $y$ to the right side of the equation.

Tap for more steps...

Step 3.3.2.1

Add $5$ to both sides of the equation.

$y = \frac{4 x}{5} - \frac{16}{5} + 5$

Step 3.3.2.2

To write $5$ as a fraction with a common denominator, multiply by $\frac{5}{5}$ .

$y = \frac{4 x}{5} - \frac{16}{5} + 5 \cdot \frac{5}{5}$

Step 3.3.2.3

Combine $5$ and $\frac{5}{5}$ .

$y = \frac{4 x}{5} - \frac{16}{5} + \frac{5 \cdot 5}{5}$

Step 3.3.2.4

Combine the numerators over the common denominator.

$y = \frac{4 x}{5} + \frac{- 16 + 5 \cdot 5}{5}$

Step 3.3.2.5

Simplify the numerator.

Tap for more steps...

Step 3.3.2.5.1

Multiply $5$ by $5$ .

$y = \frac{4 x}{5} + \frac{- 16 + 25}{5}$

Step 3.3.2.5.2

Add $- 16$ and $25$ .

$y = \frac{4 x}{5} + \frac{9}{5}$

Step 3.3.3

Reorder terms.

$y = \frac{4}{5} x + \frac{9}{5}$

Step 4