Calculus Examples

$f (x) = 10 x y - x^{3} - 5 y^{2}$

Step 1

Find the first derivative of the function.

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Step 1.1

By the Sum Rule, the derivative of $10 x y - x^{3} - 5 y^{2}$ with respect to $x$ is $\frac{d}{d x} [10 x y] + \frac{d}{d x} [- x^{3}] + \frac{d}{d x} [- 5 y^{2}]$ .

$\frac{d}{d x} [10 x y] + \frac{d}{d x} [- x^{3}] + \frac{d}{d x} [- 5 y^{2}]$

Step 1.2

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

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Step 1.2.1

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

$10 y \frac{d}{d x} [x] + \frac{d}{d x} [- x^{3}] + \frac{d}{d x} [- 5 y^{2}]$

Step 1.2.2

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

$10 y \cdot 1 + \frac{d}{d x} [- x^{3}] + \frac{d}{d x} [- 5 y^{2}]$

Step 1.2.3

Multiply $10$ by $1$ .

$10 y + \frac{d}{d x} [- x^{3}] + \frac{d}{d x} [- 5 y^{2}]$

Step 1.3

Evaluate $\frac{d}{d x} [- x^{3}]$ .

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Step 1.3.1

Since $- 1$ is constant with respect to $x$ , the derivative of $- x^{3}$ with respect to $x$ is $- \frac{d}{d x} [x^{3}]$ .

$10 y - \frac{d}{d x} [x^{3}] + \frac{d}{d x} [- 5 y^{2}]$

Step 1.3.2

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 3$ .

$10 y - (3 x^{2}) + \frac{d}{d x} [- 5 y^{2}]$

Step 1.3.3

Multiply $3$ by $- 1$ .

$10 y - 3 x^{2} + \frac{d}{d x} [- 5 y^{2}]$

Step 1.4

Since $- 5 y^{2}$ is constant with respect to $x$ , the derivative of $- 5 y^{2}$ with respect to $x$ is $0$ .

$10 y - 3 x^{2} + 0$

Step 1.5

Simplify.

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Step 1.5.1

Add $10 y - 3 x^{2}$ and $0$ .

$10 y - 3 x^{2}$

Step 1.5.2

Reorder terms.

$- 3 x^{2} + 10 y$

Step 2

Find the second derivative of the function.

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Step 2.1

By the Sum Rule, the derivative of $- 3 x^{2} + 10 y$ with respect to $x$ is $\frac{d}{d x} [- 3 x^{2}] + \frac{d}{d x} [10 y]$ .

$f'' (x) = \frac{d}{d x} (- 3 x^{2}) + \frac{d}{d x} (10 y)$

Step 2.2

Evaluate $\frac{d}{d x} [- 3 x^{2}]$ .

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Step 2.2.1

Since $- 3$ is constant with respect to $x$ , the derivative of $- 3 x^{2}$ with respect to $x$ is $- 3 \frac{d}{d x} [x^{2}]$ .

$f'' (x) = - 3 \frac{d}{d x} x^{2} + \frac{d}{d x} (10 y)$

Step 2.2.2

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

$f'' (x) = - 3 (2 x) + \frac{d}{d x} (10 y)$

Step 2.2.3

Multiply $2$ by $- 3$ .

$f'' (x) = - 6 x + \frac{d}{d x} (10 y)$

Step 2.3

Differentiate using the Constant Rule.

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Step 2.3.1

Since $10 y$ is constant with respect to $x$ , the derivative of $10 y$ with respect to $x$ is $0$ .

$f'' (x) = - 6 x + 0$

Step 2.3.2

Add $- 6 x$ and $0$ .

$f'' (x) = - 6 x$

Step 3

To find the local maximum and minimum values of the function, set the derivative equal to $0$ and solve.

$- 3 x^{2} + 10 y = 0$

Step 4

Find the first derivative.

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Step 4.1

Find the first derivative.

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Step 4.1.1

By the Sum Rule, the derivative of $10 x y - x^{3} - 5 y^{2}$ with respect to $x$ is $\frac{d}{d x} [10 x y] + \frac{d}{d x} [- x^{3}] + \frac{d}{d x} [- 5 y^{2}]$ .

$\frac{d}{d x} [10 x y] + \frac{d}{d x} [- x^{3}] + \frac{d}{d x} [- 5 y^{2}]$

Step 4.1.2

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

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Step 4.1.2.1

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

$10 y \frac{d}{d x} [x] + \frac{d}{d x} [- x^{3}] + \frac{d}{d x} [- 5 y^{2}]$

Step 4.1.2.2

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

$10 y \cdot 1 + \frac{d}{d x} [- x^{3}] + \frac{d}{d x} [- 5 y^{2}]$

Step 4.1.2.3

Multiply $10$ by $1$ .

$10 y + \frac{d}{d x} [- x^{3}] + \frac{d}{d x} [- 5 y^{2}]$

Step 4.1.3

Evaluate $\frac{d}{d x} [- x^{3}]$ .

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Step 4.1.3.1

Since $- 1$ is constant with respect to $x$ , the derivative of $- x^{3}$ with respect to $x$ is $- \frac{d}{d x} [x^{3}]$ .

$10 y - \frac{d}{d x} [x^{3}] + \frac{d}{d x} [- 5 y^{2}]$

Step 4.1.3.2

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 3$ .

$10 y - (3 x^{2}) + \frac{d}{d x} [- 5 y^{2}]$

Step 4.1.3.3

Multiply $3$ by $- 1$ .

$10 y - 3 x^{2} + \frac{d}{d x} [- 5 y^{2}]$

Step 4.1.4

Since $- 5 y^{2}$ is constant with respect to $x$ , the derivative of $- 5 y^{2}$ with respect to $x$ is $0$ .

$10 y - 3 x^{2} + 0$

Step 4.1.5

Simplify.

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Step 4.1.5.1

Add $10 y - 3 x^{2}$ and $0$ .

$10 y - 3 x^{2}$

Step 4.1.5.2

Reorder terms.

$f' (x) = - 3 x^{2} + 10 y$

Step 4.2

The first derivative of $f (x)$ with respect to $x$ is $- 3 x^{2} + 10 y$ .

$- 3 x^{2} + 10 y$

Step 5

Set the first derivative equal to $0$ then solve the equation $- 3 x^{2} + 10 y = 0$ .

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Step 5.1

Set the first derivative equal to $0$ .

$- 3 x^{2} + 10 y = 0$

Step 5.2

Subtract $10 y$ from both sides of the equation.

$- 3 x^{2} = - 10 y$

Step 5.3

Divide each term in $- 3 x^{2} = - 10 y$ by $- 3$ and simplify.

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Step 5.3.1

Divide each term in $- 3 x^{2} = - 10 y$ by $- 3$ .

$\frac{- 3 x^{2}}{- 3} = \frac{- 10 y}{- 3}$

Step 5.3.2

Simplify the left side.

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Step 5.3.2.1

Cancel the common factor of $- 3$ .

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Step 5.3.2.1.1

Cancel the common factor.

$\frac{- 3 x^{2}}{- 3} = \frac{- 10 y}{- 3}$

Step 5.3.2.1.2

Divide $x^{2}$ by $1$ .

$x^{2} = \frac{- 10 y}{- 3}$

Step 5.3.3

Simplify the right side.

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Step 5.3.3.1

Dividing two negative values results in a positive value.

$x^{2} = \frac{10 y}{3}$

Step 5.4

Take the specified root of both sides of the equation to eliminate the exponent on the left side.

$x = \pm \sqrt{\frac{10 y}{3}}$

Step 5.5

Simplify $\pm \sqrt{\frac{10 y}{3}}$ .

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Step 5.5.1

Rewrite $\sqrt{\frac{10 y}{3}}$ as $\frac{\sqrt{10 y}}{\sqrt{3}}$ .

$x = \pm \frac{\sqrt{10 y}}{\sqrt{3}}$

Step 5.5.2

Multiply $\frac{\sqrt{10 y}}{\sqrt{3}}$ by $\frac{\sqrt{3}}{\sqrt{3}}$ .

$x = \pm \frac{\sqrt{10 y}}{\sqrt{3}} \cdot \frac{\sqrt{3}}{\sqrt{3}}$

Step 5.5.3

Combine and simplify the denominator.

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Step 5.5.3.1

Multiply $\frac{\sqrt{10 y}}{\sqrt{3}}$ by $\frac{\sqrt{3}}{\sqrt{3}}$ .

$x = \pm \frac{\sqrt{10 y} \sqrt{3}}{\sqrt{3} \sqrt{3}}$

Step 5.5.3.2

Raise $\sqrt{3}$ to the power of $1$ .

$x = \pm \frac{\sqrt{10 y} \sqrt{3}}{{\sqrt{3}}^{1} \sqrt{3}}$

Step 5.5.3.3

Raise $\sqrt{3}$ to the power of $1$ .

$x = \pm \frac{\sqrt{10 y} \sqrt{3}}{{\sqrt{3}}^{1} {\sqrt{3}}^{1}}$

Step 5.5.3.4

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

$x = \pm \frac{\sqrt{10 y} \sqrt{3}}{{\sqrt{3}}^{1 + 1}}$

Step 5.5.3.5

Add $1$ and $1$ .

$x = \pm \frac{\sqrt{10 y} \sqrt{3}}{{\sqrt{3}}^{2}}$

Step 5.5.3.6

Rewrite ${\sqrt{3}}^{2}$ as $3$ .

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Step 5.5.3.6.1

Use $\sqrt[n]{a^{x}} = a^{\frac{x}{n}}$ to rewrite $\sqrt{3}$ as $3^{\frac{1}{2}}$ .

$x = \pm \frac{\sqrt{10 y} \sqrt{3}}{{(3^{\frac{1}{2}})}^{2}}$

Step 5.5.3.6.2

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

$x = \pm \frac{\sqrt{10 y} \sqrt{3}}{3^{\frac{1}{2} \cdot 2}}$

Step 5.5.3.6.3

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

$x = \pm \frac{\sqrt{10 y} \sqrt{3}}{3^{\frac{2}{2}}}$

Step 5.5.3.6.4

Cancel the common factor of $2$ .

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Step 5.5.3.6.4.1

Cancel the common factor.

$x = \pm \frac{\sqrt{10 y} \sqrt{3}}{3^{\frac{2}{2}}}$

Step 5.5.3.6.4.2

Rewrite the expression.

$x = \pm \frac{\sqrt{10 y} \sqrt{3}}{3^{1}}$

Step 5.5.3.6.5

Evaluate the exponent.

$x = \pm \frac{\sqrt{10 y} \sqrt{3}}{3}$

Step 5.5.4

Simplify the numerator.

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Step 5.5.4.1

Combine using the product rule for radicals.

$x = \pm \frac{\sqrt{10 y \cdot 3}}{3}$

Step 5.5.4.2

Multiply $3$ by $10$ .

$x = \pm \frac{\sqrt{30 y}}{3}$

Step 5.6

The complete solution is the result of both the positive and negative portions of the solution.

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Step 5.6.1

First, use the positive value of the $\pm$ to find the first solution.

$x = \frac{\sqrt{30 y}}{3}$

Step 5.6.2

Next, use the negative value of the $\pm$ to find the second solution.

$x = - \frac{\sqrt{30 y}}{3}$

Step 5.6.3

The complete solution is the result of both the positive and negative portions of the solution.

$x = \frac{\sqrt{30 y}}{3}$

$x = - \frac{\sqrt{30 y}}{3}$

$x = \frac{\sqrt{30 y}}{3}$

$x = - \frac{\sqrt{30 y}}{3}$

$x = \frac{\sqrt{30 y}}{3}$

$x = - \frac{\sqrt{30 y}}{3}$

Step 6

Find the values where the derivative is undefined.

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Step 6.1

The domain of the expression is all real numbers except where the expression is undefined. In this case, there is no real number that makes the expression undefined.

Step 7

Critical points to evaluate.

$x = \frac{\sqrt{30 y}}{3}$

$x = - \frac{\sqrt{30 y}}{3}$

Step 8

Evaluate the second derivative at $x = \frac{\sqrt{30 y}}{3}$ . If the second derivative is positive, then this is a local minimum. If it is negative, then this is a local maximum.

$- 6 \frac{\sqrt{30 y}}{3}$

Step 9

Cancel the common factor of $3$ .

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Step 9.1

Factor $3$ out of $- 6$ .

$3 (- 2) \frac{\sqrt{30 y}}{3}$

Step 9.2

Cancel the common factor.

$3 \cdot - 2 \frac{\sqrt{30 y}}{3}$

Step 9.3

Rewrite the expression.

$- 2 \sqrt{30 y}$

Step 10

Since the first derivative test failed, there are no local extrema.

No Local Extrema

Step 11