Calculus Examples

$4 x y - x^{2} y - x y^{2}$

Step 1

Write $4 x y - x^{2} y - x y^{2}$ as a function.

$f (x) = 4 x y - x^{2} y - x y^{2}$

Step 2

Find the first derivative of the function.

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Step 2.1

By the Sum Rule, the derivative of $4 x y - x^{2} y - x y^{2}$ with respect to $x$ is $\frac{d}{d x} [4 x y] + \frac{d}{d x} [- x^{2} y] + \frac{d}{d x} [- x y^{2}]$ .

$\frac{d}{d x} [4 x y] + \frac{d}{d x} [- x^{2} y] + \frac{d}{d x} [- x y^{2}]$

Step 2.2

Evaluate $\frac{d}{d x} [4 x y]$ .

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Step 2.2.1

Since $4 y$ is constant with respect to $x$ , the derivative of $4 x y$ with respect to $x$ is $4 y \frac{d}{d x} [x]$ .

$4 y \frac{d}{d x} [x] + \frac{d}{d x} [- x^{2} y] + \frac{d}{d x} [- x y^{2}]$

Step 2.2.2

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

$4 y \cdot 1 + \frac{d}{d x} [- x^{2} y] + \frac{d}{d x} [- x y^{2}]$

Step 2.2.3

Multiply $4$ by $1$ .

$4 y + \frac{d}{d x} [- x^{2} y] + \frac{d}{d x} [- x y^{2}]$

Step 2.3

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

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Step 2.3.1

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

$4 y - y \frac{d}{d x} [x^{2}] + \frac{d}{d x} [- x y^{2}]$

Step 2.3.2

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

$4 y - y (2 x) + \frac{d}{d x} [- x y^{2}]$

Step 2.3.3

Multiply $2$ by $- 1$ .

$4 y - 2 y x + \frac{d}{d x} [- x y^{2}]$

Step 2.4

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

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Step 2.4.1

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

$4 y - 2 y x - y^{2} \frac{d}{d x} [x]$

Step 2.4.2

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

$4 y - 2 y x - y^{2} \cdot 1$

Step 2.4.3

Multiply $- 1$ by $1$ .

$4 y - 2 y x - y^{2}$

Step 2.5

Reorder terms.

$- y^{2} - 2 x y + 4 y$

Step 3

Find the second derivative of the function.

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Step 3.1

Differentiate.

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Step 3.1.1

By the Sum Rule, the derivative of $- y^{2} - 2 x y + 4 y$ with respect to $x$ is $\frac{d}{d x} [- y^{2}] + \frac{d}{d x} [- 2 x y] + \frac{d}{d x} [4 y]$ .

$f'' (x) = \frac{d}{d x} (- y^{2}) + \frac{d}{d x} (- 2 x y) + \frac{d}{d x} (4 y)$

Step 3.1.2

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

$f'' (x) = 0 + \frac{d}{d x} (- 2 x y) + \frac{d}{d x} (4 y)$

Step 3.2

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

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Step 3.2.1

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

$f'' (x) = 0 - 2 y \frac{d}{d x} x + \frac{d}{d x} (4 y)$

Step 3.2.2

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

$f'' (x) = 0 - 2 y \cdot 1 + \frac{d}{d x} (4 y)$

Step 3.2.3

Multiply $- 2$ by $1$ .

$f'' (x) = 0 - 2 y + \frac{d}{d x} (4 y)$

Step 3.3

Since $4 y$ is constant with respect to $x$ , the derivative of $4 y$ with respect to $x$ is $0$ .

$f'' (x) = 0 - 2 y + 0$

Step 3.4

Combine terms.

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Step 3.4.1

Subtract $2 y$ from $0$ .

$f'' (x) = - 2 y + 0$

Step 3.4.2

Add $- 2 y$ and $0$ .

$f'' (x) = - 2 y$

Step 4

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

$- y^{2} - 2 x y + 4 y = 0$

Step 5

Find the first derivative.

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Step 5.1

Find the first derivative.

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Step 5.1.1

By the Sum Rule, the derivative of $4 x y - x^{2} y - x y^{2}$ with respect to $x$ is $\frac{d}{d x} [4 x y] + \frac{d}{d x} [- x^{2} y] + \frac{d}{d x} [- x y^{2}]$ .

$f' (x) = \frac{d}{d x} (4 x y) + \frac{d}{d x} (- x^{2} y) + \frac{d}{d x} (- x y^{2})$

Step 5.1.2

Evaluate $\frac{d}{d x} [4 x y]$ .

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Step 5.1.2.1

Since $4 y$ is constant with respect to $x$ , the derivative of $4 x y$ with respect to $x$ is $4 y \frac{d}{d x} [x]$ .

$f' (x) = 4 y \frac{d}{d x} (x) + \frac{d}{d x} (- x^{2} y) + \frac{d}{d x} (- x y^{2})$

Step 5.1.2.2

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

$f' (x) = 4 y \cdot 1 + \frac{d}{d x} (- x^{2} y) + \frac{d}{d x} (- x y^{2})$

Step 5.1.2.3

Multiply $4$ by $1$ .

$f' (x) = 4 y + \frac{d}{d x} (- x^{2} y) + \frac{d}{d x} (- x y^{2})$

Step 5.1.3

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

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Step 5.1.3.1

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

$f' (x) = 4 y - y \frac{d}{d x} x^{2} + \frac{d}{d x} (- x y^{2})$

Step 5.1.3.2

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

$f' (x) = 4 y - y (2 x) + \frac{d}{d x} (- x y^{2})$

Step 5.1.3.3

Multiply $2$ by $- 1$ .

$f' (x) = 4 y - 2 y x + \frac{d}{d x} (- x y^{2})$

Step 5.1.4

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

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Step 5.1.4.1

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

$f' (x) = 4 y - 2 y x - y^{2} \frac{d}{d x} x$

Step 5.1.4.2

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

$f' (x) = 4 y - 2 y x - y^{2} \cdot 1$

Step 5.1.4.3

Multiply $- 1$ by $1$ .

$f' (x) = 4 y - 2 y x - y^{2}$

Step 5.1.5

Reorder terms.

$f' (x) = - y^{2} - 2 x y + 4 y$

Step 5.2

The first derivative of $f (x)$ with respect to $x$ is $- y^{2} - 2 x y + 4 y$ .

$- y^{2} - 2 x y + 4 y$

Step 6

Set the first derivative equal to $0$ then solve the equation $- y^{2} - 2 x y + 4 y = 0$ .

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Step 6.1

Set the first derivative equal to $0$ .

$- y^{2} - 2 x y + 4 y = 0$

Step 6.2

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

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Step 6.2.1

Add $y^{2}$ to both sides of the equation.

$- 2 x y + 4 y = y^{2}$

Step 6.2.2

Subtract $4 y$ from both sides of the equation.

$- 2 x y = y^{2} - 4 y$

Step 6.3

Divide each term in $- 2 x y = y^{2} - 4 y$ by $- 2 y$ and simplify.

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Step 6.3.1

Divide each term in $- 2 x y = y^{2} - 4 y$ by $- 2 y$ .

$\frac{- 2 x y}{- 2 y} = \frac{y^{2}}{- 2 y} + \frac{- 4 y}{- 2 y}$

Step 6.3.2

Simplify the left side.

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Step 6.3.2.1

Cancel the common factor of $- 2$ .

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Step 6.3.2.1.1

Cancel the common factor.

$\frac{- 2 x y}{- 2 y} = \frac{y^{2}}{- 2 y} + \frac{- 4 y}{- 2 y}$

Step 6.3.2.1.2

Rewrite the expression.

$\frac{x y}{y} = \frac{y^{2}}{- 2 y} + \frac{- 4 y}{- 2 y}$

Step 6.3.2.2

Cancel the common factor of $y$ .

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Step 6.3.2.2.1

Cancel the common factor.

$\frac{x y}{y} = \frac{y^{2}}{- 2 y} + \frac{- 4 y}{- 2 y}$

Step 6.3.2.2.2

Divide $x$ by $1$ .

$x = \frac{y^{2}}{- 2 y} + \frac{- 4 y}{- 2 y}$

Step 6.3.3

Simplify the right side.

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Step 6.3.3.1

Simplify each term.

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Step 6.3.3.1.1

Cancel the common factor of $y^{2}$ and $y$ .

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Step 6.3.3.1.1.1

Factor $y$ out of $y^{2}$ .

$x = \frac{y \cdot y}{- 2 y} + \frac{- 4 y}{- 2 y}$

Step 6.3.3.1.1.2

Cancel the common factors.

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Step 6.3.3.1.1.2.1

Factor $y$ out of $- 2 y$ .

$x = \frac{y \cdot y}{y \cdot - 2} + \frac{- 4 y}{- 2 y}$

Step 6.3.3.1.1.2.2

Cancel the common factor.

$x = \frac{y \cdot y}{y \cdot - 2} + \frac{- 4 y}{- 2 y}$

Step 6.3.3.1.1.2.3

Rewrite the expression.

$x = \frac{y}{- 2} + \frac{- 4 y}{- 2 y}$

Step 6.3.3.1.2

Move the negative in front of the fraction.

$x = - \frac{y}{2} + \frac{- 4 y}{- 2 y}$

Step 6.3.3.1.3

Cancel the common factor of $- 4$ and $- 2$ .

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Step 6.3.3.1.3.1

Factor $- 2$ out of $- 4 y$ .

$x = - \frac{y}{2} + \frac{- 2 (2 y)}{- 2 y}$

Step 6.3.3.1.3.2

Cancel the common factors.

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Step 6.3.3.1.3.2.1

Factor $- 2$ out of $- 2 y$ .

$x = - \frac{y}{2} + \frac{- 2 (2 y)}{- 2 (y)}$

Step 6.3.3.1.3.2.2

Cancel the common factor.

$x = - \frac{y}{2} + \frac{- 2 (2 y)}{- 2 y}$

Step 6.3.3.1.3.2.3

Rewrite the expression.

$x = - \frac{y}{2} + \frac{2 y}{y}$

Step 6.3.3.1.4

Cancel the common factor of $y$ .

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Step 6.3.3.1.4.1

Cancel the common factor.

$x = - \frac{y}{2} + \frac{2 y}{y}$

Step 6.3.3.1.4.2

Divide $2$ by $1$ .

$x = - \frac{y}{2} + 2$

Step 7

Find the values where the derivative is undefined.

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Step 7.1

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 8

Critical points to evaluate.

$x = - \frac{y}{2} + 2$

Step 9

Evaluate the second derivative at $x = - \frac{y}{2} + 2$ . If the second derivative is positive, then this is a local minimum. If it is negative, then this is a local maximum.

$- 2 y$

Step 10

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

No Local Extrema

Step 11