Calculus Examples

$lim_{x \to 9} \frac{x^{2} - 10 x + 9}{\sqrt{x} - 3}$

Step 1

Apply L'Hospital's rule.

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Step 1.1

Evaluate the limit of the numerator and the limit of the denominator.

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Step 1.1.1

Take the limit of the numerator and the limit of the denominator.

$\frac{lim_{x \to 9} x^{2} - 10 x + 9}{lim_{x \to 9} \sqrt{x} - 3}$

Step 1.1.2

Evaluate the limit of the numerator.

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Step 1.1.2.1

Split the limit using the Sum of Limits Rule on the limit as $x$ approaches $9$ .

$\frac{lim_{x \to 9} x^{2} - lim_{x \to 9} 10 x + lim_{x \to 9} 9}{lim_{x \to 9} \sqrt{x} - 3}$

Step 1.1.2.2

Move the exponent $2$ from $x^{2}$ outside the limit using the Limits Power Rule.

$\frac{{(lim_{x \to 9} x)}^{2} - lim_{x \to 9} 10 x + lim_{x \to 9} 9}{lim_{x \to 9} \sqrt{x} - 3}$

Step 1.1.2.3

Move the term $10$ outside of the limit because it is constant with respect to $x$ .

$\frac{{(lim_{x \to 9} x)}^{2} - 10 lim_{x \to 9} x + lim_{x \to 9} 9}{lim_{x \to 9} \sqrt{x} - 3}$

Step 1.1.2.4

Evaluate the limit of $9$ which is constant as $x$ approaches $9$ .

$\frac{{(lim_{x \to 9} x)}^{2} - 10 lim_{x \to 9} x + 9}{lim_{x \to 9} \sqrt{x} - 3}$

Step 1.1.2.5

Evaluate the limits by plugging in $9$ for all occurrences of $x$ .

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Step 1.1.2.5.1

Evaluate the limit of $x$ by plugging in $9$ for $x$ .

$\frac{9^{2} - 10 lim_{x \to 9} x + 9}{lim_{x \to 9} \sqrt{x} - 3}$

Step 1.1.2.5.2

Evaluate the limit of $x$ by plugging in $9$ for $x$ .

$\frac{9^{2} - 10 \cdot 9 + 9}{lim_{x \to 9} \sqrt{x} - 3}$

Step 1.1.2.6

Simplify the answer.

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Step 1.1.2.6.1

Simplify each term.

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Step 1.1.2.6.1.1

Raise $9$ to the power of $2$ .

$\frac{81 - 10 \cdot 9 + 9}{lim_{x \to 9} \sqrt{x} - 3}$

Step 1.1.2.6.1.2

Multiply $- 10$ by $9$ .

$\frac{81 - 90 + 9}{lim_{x \to 9} \sqrt{x} - 3}$

Step 1.1.2.6.2

Subtract $90$ from $81$ .

$\frac{- 9 + 9}{lim_{x \to 9} \sqrt{x} - 3}$

Step 1.1.2.6.3

Add $- 9$ and $9$ .

$\frac{0}{lim_{x \to 9} \sqrt{x} - 3}$

Step 1.1.3

Evaluate the limit of the denominator.

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Step 1.1.3.1

Evaluate the limit.

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Step 1.1.3.1.1

Split the limit using the Sum of Limits Rule on the limit as $x$ approaches $9$ .

$\frac{0}{lim_{x \to 9} \sqrt{x} - lim_{x \to 9} 3}$

Step 1.1.3.1.2

Move the limit under the radical sign.

$\frac{0}{\sqrt{lim_{x \to 9} x} - lim_{x \to 9} 3}$

Step 1.1.3.1.3

Evaluate the limit of $3$ which is constant as $x$ approaches $9$ .

$\frac{0}{\sqrt{lim_{x \to 9} x} - 1 \cdot 3}$

Step 1.1.3.2

Evaluate the limit of $x$ by plugging in $9$ for $x$ .

$\frac{0}{\sqrt{9} - 1 \cdot 3}$

Step 1.1.3.3

Simplify the answer.

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Step 1.1.3.3.1

Simplify each term.

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Step 1.1.3.3.1.1

Rewrite $9$ as $3^{2}$ .

$\frac{0}{\sqrt{3^{2}} - 1 \cdot 3}$

Step 1.1.3.3.1.2

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

$\frac{0}{3 - 1 \cdot 3}$

Step 1.1.3.3.1.3

Multiply $- 1$ by $3$ .

$\frac{0}{3 - 3}$

Step 1.1.3.3.2

Subtract $3$ from $3$ .

$\frac{0}{0}$

Step 1.1.3.3.3

The expression contains a division by $0$ . The expression is undefined.

Undefined

$\frac{0}{0}$

Step 1.1.3.4

The expression contains a division by $0$ . The expression is undefined.

Undefined

$\frac{0}{0}$

Step 1.1.4

The expression contains a division by $0$ . The expression is undefined.

Undefined

$\frac{0}{0}$

Step 1.2

Since $\frac{0}{0}$ is of indeterminate form, apply L'Hospital's Rule. L'Hospital's Rule states that the limit of a quotient of functions is equal to the limit of the quotient of their derivatives.

$lim_{x \to 9} \frac{x^{2} - 10 x + 9}{\sqrt{x} - 3} = lim_{x \to 9} \frac{\frac{d}{d x} [x^{2} - 10 x + 9]}{\frac{d}{d x} [\sqrt{x} - 3]}$

Step 1.3

Find the derivative of the numerator and denominator.

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Step 1.3.1

Differentiate the numerator and denominator.

$lim_{x \to 9} \frac{\frac{d}{d x} [x^{2} - 10 x + 9]}{\frac{d}{d x} [\sqrt{x} - 3]}$

Step 1.3.2

By the Sum Rule, the derivative of $x^{2} - 10 x + 9$ with respect to $x$ is $\frac{d}{d x} [x^{2}] + \frac{d}{d x} [- 10 x] + \frac{d}{d x} [9]$ .

$lim_{x \to 9} \frac{\frac{d}{d x} [x^{2}] + \frac{d}{d x} [- 10 x] + \frac{d}{d x} [9]}{\frac{d}{d x} [\sqrt{x} - 3]}$

Step 1.3.3

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

$lim_{x \to 9} \frac{2 x + \frac{d}{d x} [- 10 x] + \frac{d}{d x} [9]}{\frac{d}{d x} [\sqrt{x} - 3]}$

Step 1.3.4

Evaluate $\frac{d}{d x} [- 10 x]$ .

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Step 1.3.4.1

Since $- 10$ is constant with respect to $x$ , the derivative of $- 10 x$ with respect to $x$ is $- 10 \frac{d}{d x} [x]$ .

$lim_{x \to 9} \frac{2 x - 10 \frac{d}{d x} [x] + \frac{d}{d x} [9]}{\frac{d}{d x} [\sqrt{x} - 3]}$

Step 1.3.4.2

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

$lim_{x \to 9} \frac{2 x - 10 \cdot 1 + \frac{d}{d x} [9]}{\frac{d}{d x} [\sqrt{x} - 3]}$

Step 1.3.4.3

Multiply $- 10$ by $1$ .

$lim_{x \to 9} \frac{2 x - 10 + \frac{d}{d x} [9]}{\frac{d}{d x} [\sqrt{x} - 3]}$

Step 1.3.5

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

$lim_{x \to 9} \frac{2 x - 10 + 0}{\frac{d}{d x} [\sqrt{x} - 3]}$

Step 1.3.6

Add $2 x - 10$ and $0$ .

$lim_{x \to 9} \frac{2 x - 10}{\frac{d}{d x} [\sqrt{x} - 3]}$

Step 1.3.7

By the Sum Rule, the derivative of $\sqrt{x} - 3$ with respect to $x$ is $\frac{d}{d x} [\sqrt{x}] + \frac{d}{d x} [- 3]$ .

$lim_{x \to 9} \frac{2 x - 10}{\frac{d}{d x} [\sqrt{x}] + \frac{d}{d x} [- 3]}$

Step 1.3.8

Evaluate $\frac{d}{d x} [\sqrt{x}]$ .

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Step 1.3.8.1

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

$lim_{x \to 9} \frac{2 x - 10}{\frac{d}{d x} [x^{\frac{1}{2}}] + \frac{d}{d x} [- 3]}$

Step 1.3.8.2

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

$lim_{x \to 9} \frac{2 x - 10}{\frac{1}{2} x^{\frac{1}{2} - 1} + \frac{d}{d x} [- 3]}$

Step 1.3.8.3

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

$lim_{x \to 9} \frac{2 x - 10}{\frac{1}{2} x^{\frac{1}{2} - 1 \cdot \frac{2}{2}} + \frac{d}{d x} [- 3]}$

Step 1.3.8.4

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

$lim_{x \to 9} \frac{2 x - 10}{\frac{1}{2} x^{\frac{1}{2} + \frac{- 1 \cdot 2}{2}} + \frac{d}{d x} [- 3]}$

Step 1.3.8.5

Combine the numerators over the common denominator.

$lim_{x \to 9} \frac{2 x - 10}{\frac{1}{2} x^{\frac{1 - 1 \cdot 2}{2}} + \frac{d}{d x} [- 3]}$

Step 1.3.8.6

Simplify the numerator.

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Step 1.3.8.6.1

Multiply $- 1$ by $2$ .

$lim_{x \to 9} \frac{2 x - 10}{\frac{1}{2} x^{\frac{1 - 2}{2}} + \frac{d}{d x} [- 3]}$

Step 1.3.8.6.2

Subtract $2$ from $1$ .

$lim_{x \to 9} \frac{2 x - 10}{\frac{1}{2} x^{\frac{- 1}{2}} + \frac{d}{d x} [- 3]}$

Step 1.3.8.7

Move the negative in front of the fraction.

$lim_{x \to 9} \frac{2 x - 10}{\frac{1}{2} x^{- \frac{1}{2}} + \frac{d}{d x} [- 3]}$

Step 1.3.9

Since $- 3$ is constant with respect to $x$ , the derivative of $- 3$ with respect to $x$ is $0$ .

$lim_{x \to 9} \frac{2 x - 10}{\frac{1}{2} x^{- \frac{1}{2}} + 0}$

Step 1.3.10

Simplify.

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Step 1.3.10.1

Rewrite the expression using the negative exponent rule $b^{- n} = \frac{1}{b^{n}}$ .

$lim_{x \to 9} \frac{2 x - 10}{\frac{1}{2} \cdot \frac{1}{x^{\frac{1}{2}}} + 0}$

Step 1.3.10.2

Combine terms.

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Step 1.3.10.2.1

Multiply $\frac{1}{2}$ by $\frac{1}{x^{\frac{1}{2}}}$ .

$lim_{x \to 9} \frac{2 x - 10}{\frac{1}{2 x^{\frac{1}{2}}} + 0}$

Step 1.3.10.2.2

Add $\frac{1}{2 x^{\frac{1}{2}}}$ and $0$ .

$lim_{x \to 9} \frac{2 x - 10}{\frac{1}{2 x^{\frac{1}{2}}}}$

Step 1.4

Multiply the numerator by the reciprocal of the denominator.

$lim_{x \to 9} (2 x - 10) (2 x^{\frac{1}{2}})$

Step 1.5

Rewrite $x^{\frac{1}{2}}$ as $\sqrt{x}$ .

$lim_{x \to 9} (2 x - 10) (2 \sqrt{x})$

Step 2

Evaluate the limit.

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Step 2.1

Move the term $2$ outside of the limit because it is constant with respect to $x$ .

$2 lim_{x \to 9} (2 x - 10) \sqrt{x}$

Step 2.2

Split the limit using the Product of Limits Rule on the limit as $x$ approaches $9$ .

$2 lim_{x \to 9} 2 x - 10 \cdot lim_{x \to 9} \sqrt{x}$

Step 2.3

Split the limit using the Sum of Limits Rule on the limit as $x$ approaches $9$ .

$2 (lim_{x \to 9} 2 x - lim_{x \to 9} 10) \cdot lim_{x \to 9} \sqrt{x}$

Step 2.4

Move the term $2$ outside of the limit because it is constant with respect to $x$ .

$2 (2 lim_{x \to 9} x - lim_{x \to 9} 10) \cdot lim_{x \to 9} \sqrt{x}$

Step 2.5

Evaluate the limit of $10$ which is constant as $x$ approaches $9$ .

$2 (2 lim_{x \to 9} x - 1 \cdot 10) \cdot lim_{x \to 9} \sqrt{x}$

Step 2.6

Move the limit under the radical sign.

$2 (2 lim_{x \to 9} x - 1 \cdot 10) \cdot \sqrt{lim_{x \to 9} x}$

Step 3

Evaluate the limits by plugging in $9$ for all occurrences of $x$ .

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Step 3.1

Evaluate the limit of $x$ by plugging in $9$ for $x$ .

$2 (2 \cdot 9 - 1 \cdot 10) \cdot \sqrt{lim_{x \to 9} x}$

Step 3.2

Evaluate the limit of $x$ by plugging in $9$ for $x$ .

$2 (2 \cdot 9 - 1 \cdot 10) \cdot \sqrt{9}$

Step 4

Simplify the answer.

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Step 4.1

Simplify each term.

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Step 4.1.1

Multiply $2$ by $9$ .

$2 (18 - 1 \cdot 10) \cdot \sqrt{9}$

Step 4.1.2

Multiply $- 1$ by $10$ .

$2 (18 - 10) \cdot \sqrt{9}$

Step 4.2

Subtract $10$ from $18$ .

$2 \cdot 8 \cdot \sqrt{9}$

Step 4.3

Multiply $2$ by $8$ .

$16 \cdot \sqrt{9}$

Step 4.4

Rewrite $9$ as $3^{2}$ .

$16 \cdot \sqrt{3^{2}}$

Step 4.5

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

$16 \cdot 3$

Step 4.6

Multiply $16$ by $3$ .

$48$