Calculus Examples

$lim_{x \to \infty} \sqrt{x^{4} - 9 x^{2}} - x^{2}$

Step 1

Multiply to rationalize the numerator.

$lim_{x \to \infty} \frac{(\sqrt{x^{4} - 9 x^{2}} - x^{2}) (\sqrt{x^{4} - 9 x^{2}} + x^{2})}{\sqrt{x^{4} - 9 x^{2}} + x^{2}}$

Step 2

Simplify.

Tap for more steps...

Step 2.1

Expand the numerator using the FOIL method.

$lim_{x \to \infty} \frac{{\sqrt{x^{4} - 9 x^{2}}}^{2} + \sqrt{x^{4} - 9 x^{2}} x^{2} + \sqrt{x^{4} - 9 x^{2}} (- x^{2}) - x^{4}}{\sqrt{x^{4} - 9 x^{2}} + x^{2}}$

Step 2.2

Simplify.

Tap for more steps...

Step 2.2.1

Subtract $x^{4}$ from $x^{4}$ .

$lim_{x \to \infty} \frac{- 9 x^{2} + 0}{\sqrt{x^{4} - 9 x^{2}} + x^{2}}$

Step 2.2.2

Add $- 9 x^{2}$ and $0$ .

$lim_{x \to \infty} \frac{- 9 x^{2}}{\sqrt{x^{4} - 9 x^{2}} + x^{2}}$

Step 3

Evaluate the limit.

Tap for more steps...

Step 3.1

Simplify each term.

Tap for more steps...

Step 3.1.1

Factor $x^{2}$ out of $x^{4} - 9 x^{2}$ .

Tap for more steps...

Step 3.1.1.1

Factor $x^{2}$ out of $x^{4}$ .

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

Step 3.1.1.2

Factor $x^{2}$ out of $- 9 x^{2}$ .

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

Step 3.1.1.3

Factor $x^{2}$ out of $x^{2} x^{2} + x^{2} \cdot - 9$ .

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

Step 3.1.2

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

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

Step 3.1.3

Since both terms are perfect squares, factor using the difference of squares formula, $a^{2} - b^{2} = (a + b) (a - b)$ where $a = x$ and $b = 3$ .

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

Step 3.1.4

Add parentheses.

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

Step 3.1.5

Pull terms out from under the radical.

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

Step 3.2

Move the term $- 9$ outside of the limit because it is constant with respect to $x$ .

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

Step 4

Divide the numerator and denominator by the highest power of $x$ in the denominator, which is $x^{2}$ .

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

Step 5

Simplify terms.

Tap for more steps...

Step 5.1

Cancel the common factor of $x^{2}$ .

Tap for more steps...

Step 5.1.1

Cancel the common factor.

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

Step 5.1.2

Rewrite the expression.

$- 9 lim_{x \to \infty} \frac{1}{\frac{x \sqrt{(x + 3) (x - 3)}}{x^{2}} + \frac{x^{2}}{x^{2}}}$

Step 5.2

Simplify each term.

$- 9 lim_{x \to \infty} \frac{1}{\frac{\sqrt{(x + 3) (x - 3)}}{x} + 1}$

Step 6

Expand $(x + 3) (x - 3)$ using the FOIL Method.

Tap for more steps...

Step 6.1

Apply the distributive property.

$- 9 lim_{x \to \infty} \frac{1}{\frac{\sqrt{x (x - 3) + 3 (x - 3)}}{x} + 1}$

Step 6.2

Apply the distributive property.

$- 9 lim_{x \to \infty} \frac{1}{\frac{\sqrt{x \cdot x + x \cdot - 3 + 3 (x - 3)}}{x} + 1}$

Step 6.3

Apply the distributive property.

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

Step 7

Simplify terms.

Tap for more steps...

Step 7.1

Combine the opposite terms in $x \cdot x + x \cdot - 3 + 3 x + 3 \cdot - 3$ .

Tap for more steps...

Step 7.1.1

Reorder the factors in the terms $x \cdot - 3$ and $3 x$ .

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

Step 7.1.2

Add $- 3 x$ and $3 x$ .

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

Step 7.1.3

Add $x \cdot x$ and $0$ .

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

Step 7.2

Simplify each term.

Tap for more steps...

Step 7.2.1

Multiply $x$ by $x$ .

$- 9 lim_{x \to \infty} \frac{1}{\frac{\sqrt{x^{2} + 3 \cdot - 3}}{x} + 1}$

Step 7.2.2

Multiply $3$ by $- 3$ .

$- 9 lim_{x \to \infty} \frac{1}{\frac{\sqrt{x^{2} - 9}}{x} + 1}$

Step 8

Split the limit using the Limits Quotient Rule on the limit as $x$ approaches $\infty$ .

$- 9 \frac{lim_{x \to \infty} 1}{lim_{x \to \infty} \frac{\sqrt{x^{2} - 9}}{x} + 1}$

Step 9

Evaluate the limit of $1$ which is constant as $x$ approaches $\infty$ .

$- 9 \frac{1}{lim_{x \to \infty} \frac{\sqrt{x^{2} - 9}}{x} + 1}$

Step 10

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

$- 9 \frac{1}{lim_{x \to \infty} \frac{\sqrt{x^{2} - 9}}{x} + lim_{x \to \infty} 1}$

Step 11

Simplify.

Tap for more steps...

Step 11.1

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

$- 9 \frac{1}{lim_{x \to \infty} \frac{\sqrt{x^{2} - 3^{2}}}{x} + lim_{x \to \infty} 1}$

Step 11.2

Since both terms are perfect squares, factor using the difference of squares formula, $a^{2} - b^{2} = (a + b) (a - b)$ where $a = x$ and $b = 3$ .

$- 9 \frac{1}{lim_{x \to \infty} \frac{\sqrt{(x + 3) (x - 3)}}{x} + lim_{x \to \infty} 1}$

Step 12

Divide the numerator and denominator by the highest power of $x$ in the denominator, which is $\sqrt{x^{2}} = x$ .

$- 9 \frac{1}{lim_{x \to \infty} \frac{\sqrt{\frac{(x + 3) (x - 3)}{x^{2}}}}{\frac{x}{x}} + lim_{x \to \infty} 1}$

Step 13

Evaluate the limit.

Tap for more steps...

Step 13.1

Cancel the common factor of $x$ .

$- 9 \frac{1}{lim_{x \to \infty} \frac{\sqrt{\frac{(x + 3) (x - 3)}{x^{2}}}}{1} + lim_{x \to \infty} 1}$

Step 13.2

Split the limit using the Limits Quotient Rule on the limit as $x$ approaches $\infty$ .

$- 9 \frac{1}{\frac{lim_{x \to \infty} \sqrt{\frac{(x + 3) (x - 3)}{x^{2}}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 13.3

Move the limit under the radical sign.

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{(x + 3) (x - 3)}{x^{2}}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14

Apply L'Hospital's rule.

Tap for more steps...

Step 14.1

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

Tap for more steps...

Step 14.1.1

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

$- 9 \frac{1}{\frac{\sqrt{\frac{lim_{x \to \infty} (x + 3) (x - 3)}{lim_{x \to \infty} x^{2}}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.1.2

Evaluate the limit of the numerator.

Tap for more steps...

Step 14.1.2.1

Apply the distributive property.

$- 9 \frac{1}{\frac{\sqrt{\frac{lim_{x \to \infty} x (x - 3) + 3 (x - 3)}{lim_{x \to \infty} x^{2}}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.1.2.2

Apply the distributive property.

$- 9 \frac{1}{\frac{\sqrt{\frac{lim_{x \to \infty} x \cdot x + x \cdot - 3 + 3 (x - 3)}{lim_{x \to \infty} x^{2}}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.1.2.3

Apply the distributive property.

$- 9 \frac{1}{\frac{\sqrt{\frac{lim_{x \to \infty} x \cdot x + x \cdot - 3 + 3 x + 3 \cdot - 3}{lim_{x \to \infty} x^{2}}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.1.2.4

Reorder $x$ and $- 3$ .

$- 9 \frac{1}{\frac{\sqrt{\frac{lim_{x \to \infty} x \cdot x - 3 \cdot x + 3 x + 3 \cdot - 3}{lim_{x \to \infty} x^{2}}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.1.2.5

Raise $x$ to the power of $1$ .

$- 9 \frac{1}{\frac{\sqrt{\frac{lim_{x \to \infty} x^{1} x - 3 x + 3 x + 3 \cdot - 3}{lim_{x \to \infty} x^{2}}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.1.2.6

Raise $x$ to the power of $1$ .

$- 9 \frac{1}{\frac{\sqrt{\frac{lim_{x \to \infty} x^{1} x^{1} - 3 x + 3 x + 3 \cdot - 3}{lim_{x \to \infty} x^{2}}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.1.2.7

Use the power rule $a^{m} a^{n} = a^{m + n}$ to combine exponents.

$- 9 \frac{1}{\frac{\sqrt{\frac{lim_{x \to \infty} x^{1 + 1} - 3 x + 3 x + 3 \cdot - 3}{lim_{x \to \infty} x^{2}}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.1.2.8

Simplify by adding terms.

Tap for more steps...

Step 14.1.2.8.1

Add $1$ and $1$ .

$- 9 \frac{1}{\frac{\sqrt{\frac{lim_{x \to \infty} x^{2} - 3 x + 3 x + 3 \cdot - 3}{lim_{x \to \infty} x^{2}}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.1.2.8.2

Multiply $3$ by $- 3$ .

$- 9 \frac{1}{\frac{\sqrt{\frac{lim_{x \to \infty} x^{2} - 3 x + 3 x - 9}{lim_{x \to \infty} x^{2}}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.1.2.8.3

Add $- 3 x$ and $3 x$ .

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

Step 14.1.2.8.4

Subtract $9$ from $0$ .

$- 9 \frac{1}{\frac{\sqrt{\frac{lim_{x \to \infty} x^{2} - 9}{lim_{x \to \infty} x^{2}}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.1.2.9

The limit at infinity of a polynomial whose leading coefficient is positive is infinity.

$- 9 \frac{1}{\frac{\sqrt{\frac{\infty}{lim_{x \to \infty} x^{2}}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.1.3

The limit at infinity of a polynomial whose leading coefficient is positive is infinity.

$- 9 \frac{1}{\frac{\sqrt{\frac{\infty}{\infty}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.1.4

Infinity divided by infinity is undefined.

Undefined

$- 9 \frac{1}{\frac{\sqrt{\frac{\infty}{\infty}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.2

Since $\frac{\infty}{\infty}$ 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.

$lim_{x \to \infty} \frac{(x + 3) (x - 3)}{x^{2}} = lim_{x \to \infty} \frac{\frac{d}{d x} [(x + 3) (x - 3)]}{\frac{d}{d x} [x^{2}]}$

Step 14.3

Find the derivative of the numerator and denominator.

Tap for more steps...

Step 14.3.1

Differentiate the numerator and denominator.

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{\frac{d}{d x} [(x + 3) (x - 3)]}{\frac{d}{d x} [x^{2}]}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.3.2

Differentiate using the Product Rule which states that $\frac{d}{d x} [f (x) g (x)]$ is $f (x) \frac{d}{d x} [g (x)] + g (x) \frac{d}{d x} [f (x)]$ where $f (x) = x + 3$ and $g (x) = x - 3$ .

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{(x + 3) \frac{d}{d x} [x - 3] + (x - 3) \frac{d}{d x} [x + 3]}{\frac{d}{d x} [x^{2}]}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.3.3

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

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{(x + 3) (\frac{d}{d x} [x] + \frac{d}{d x} [- 3]) + (x - 3) \frac{d}{d x} [x + 3]}{\frac{d}{d x} [x^{2}]}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.3.4

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

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{(x + 3) (1 + \frac{d}{d x} [- 3]) + (x - 3) \frac{d}{d x} [x + 3]}{\frac{d}{d x} [x^{2}]}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.3.5

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

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{(x + 3) (1 + 0) + (x - 3) \frac{d}{d x} [x + 3]}{\frac{d}{d x} [x^{2}]}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.3.6

Add $1$ and $0$ .

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{(x + 3) \cdot 1 + (x - 3) \frac{d}{d x} [x + 3]}{\frac{d}{d x} [x^{2}]}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.3.7

Multiply $x + 3$ by $1$ .

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{x + 3 + (x - 3) \frac{d}{d x} [x + 3]}{\frac{d}{d x} [x^{2}]}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.3.8

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

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{x + 3 + (x - 3) (\frac{d}{d x} [x] + \frac{d}{d x} [3])}{\frac{d}{d x} [x^{2}]}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.3.9

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

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{x + 3 + (x - 3) (1 + \frac{d}{d x} [3])}{\frac{d}{d x} [x^{2}]}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.3.10

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

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{x + 3 + (x - 3) (1 + 0)}{\frac{d}{d x} [x^{2}]}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.3.11

Add $1$ and $0$ .

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{x + 3 + (x - 3) \cdot 1}{\frac{d}{d x} [x^{2}]}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.3.12

Multiply $x - 3$ by $1$ .

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

Step 14.3.13

Add $x$ and $x$ .

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

Step 14.3.14

Subtract $3$ from $3$ .

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{2 x + 0}{\frac{d}{d x} [x^{2}]}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.3.15

Add $2 x$ and $0$ .

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

Step 14.3.16

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

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{2 x}{2 x}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.4

Reduce.

Tap for more steps...

Step 14.4.1

Cancel the common factor of $2$ .

Tap for more steps...

Step 14.4.1.1

Cancel the common factor.

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{2 x}{2 x}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.4.1.2

Rewrite the expression.

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{x}{x}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.4.2

Cancel the common factor of $x$ .

Tap for more steps...

Step 14.4.2.1

Cancel the common factor.

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} \frac{x}{x}}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 14.4.2.2

Rewrite the expression.

$- 9 \frac{1}{\frac{\sqrt{lim_{x \to \infty} 1}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 15

Evaluate the limit.

Tap for more steps...

Step 15.1

Evaluate the limit of $1$ which is constant as $x$ approaches $\infty$ .

$- 9 \frac{1}{\frac{\sqrt{1}}{lim_{x \to \infty} 1} + lim_{x \to \infty} 1}$

Step 15.2

Evaluate the limit of $1$ which is constant as $x$ approaches $\infty$ .

$- 9 \frac{1}{\frac{\sqrt{1}}{1} + lim_{x \to \infty} 1}$

Step 15.3

Evaluate the limit of $1$ which is constant as $x$ approaches $\infty$ .

$- 9 \frac{1}{\frac{\sqrt{1}}{1} + 1}$

Step 15.4

Simplify the answer.

Tap for more steps...

Step 15.4.1

Divide $\sqrt{1}$ by $1$ .

$- 9 \frac{1}{\sqrt{1} + 1}$

Step 15.4.2

Simplify the denominator.

Tap for more steps...

Step 15.4.2.1

Any root of $1$ is $1$ .

$- 9 \frac{1}{1 + 1}$

Step 15.4.2.2

Add $1$ and $1$ .

$- 9 (\frac{1}{2})$

Step 15.4.3

Combine $- 9$ and $\frac{1}{2}$ .

$\frac{- 9}{2}$

Step 15.4.4

Move the negative in front of the fraction.

$- \frac{9}{2}$

Step 16

The result can be shown in multiple forms.

Exact Form:

$- \frac{9}{2}$

Decimal Form:

$- 4.5$