Calculus Examples

$3 x^{2} y d x + (y + x^{3}) d y = 0$

Step 1

Find $\frac{\partial M}{\partial y}$ where $M (x, y) = 3 x^{2} y$ .

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Step 1.1

Differentiate $M$ with respect to $y$ .

$\frac{\partial M}{\partial y} = \frac{d}{d y} [3 x^{2} y]$

Step 1.2

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

$\frac{\partial M}{\partial y} = 3 x^{2} \frac{d}{d y} [y]$

Step 1.3

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

$\frac{\partial M}{\partial y} = 3 x^{2} \cdot 1$

Step 1.4

Multiply $3$ by $1$ .

$\frac{\partial M}{\partial y} = 3 x^{2}$

Step 2

Find $\frac{\partial N}{\partial x}$ where $N (x, y) = y + x^{3}$ .

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Step 2.1

Differentiate $N$ with respect to $x$ .

$\frac{\partial N}{\partial x} = \frac{d}{d x} [y + x^{3}]$

Step 2.2

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

$\frac{\partial N}{\partial x} = \frac{d}{d x} [y] + \frac{d}{d x} [x^{3}]$

Step 2.3

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

$\frac{\partial N}{\partial x} = 0 + \frac{d}{d x} [x^{3}]$

Step 2.4

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

$\frac{\partial N}{\partial x} = 0 + 3 x^{2}$

Step 2.5

Add $0$ and $3 x^{2}$ .

$\frac{\partial N}{\partial x} = 3 x^{2}$

Step 3

Check that $\frac{\partial M}{\partial y} = \frac{\partial N}{\partial x}$ .

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Step 3.1

Substitute $3 x^{2}$ for $\frac{\partial M}{\partial y}$ and $3 x^{2}$ for $\frac{\partial N}{\partial x}$ .

$3 x^{2} = 3 x^{2}$

Step 3.2

Since the two sides have been shown to be equivalent, the equation is an identity.

$3 x^{2} = 3 x^{2}$ is an identity.

Step 4

Set $f (x, y)$ equal to the integral of $M (x, y)$ .

$f (x, y) = \int 3 x^{2} y d x$

Step 5

Integrate $M (x, y) = 3 x^{2} y$ to find $f (x, y)$ .

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Step 5.1

Since $3 y$ is constant with respect to $x$ , move $3 y$ out of the integral.

$f (x, y) = 3 y \int x^{2} d x$

Step 5.2

By the Power Rule, the integral of $x^{2}$ with respect to $x$ is $\frac{1}{3} x^{3}$ .

$f (x, y) = 3 y (\frac{1}{3} x^{3} + C)$

Step 5.3

Simplify the answer.

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Step 5.3.1

Rewrite $3 y (\frac{1}{3} x^{3} + C)$ as $3 y \frac{1}{3} x^{3} + C$ .

$f (x, y) = 3 y \frac{1}{3} x^{3} + C$

Step 5.3.2

Simplify.

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Step 5.3.2.1

Combine $3$ and $\frac{1}{3}$ .

$f (x, y) = y \frac{3}{3} x^{3} + C$

Step 5.3.2.2

Combine $y$ and $\frac{3}{3}$ .

$f (x, y) = \frac{y \cdot 3}{3} x^{3} + C$

Step 5.3.2.3

Move $3$ to the left of $y$ .

$f (x, y) = \frac{3 \cdot y}{3} x^{3} + C$

Step 5.3.2.4

Multiply $3$ by $y$ .

$f (x, y) = \frac{3 y}{3} x^{3} + C$

Step 5.3.2.5

Cancel the common factor of $3$ .

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Step 5.3.2.5.1

Cancel the common factor.

$f (x, y) = \frac{3 y}{3} x^{3} + C$

Step 5.3.2.5.2

Divide $y$ by $1$ .

$f (x, y) = y x^{3} + C$

Step 6

Since the integral of $g (y)$ will contain an integration constant, we can replace $C$ with $g (y)$ .

$f (x, y) = y x^{3} + g (y)$

Step 7

Set $\frac{\partial f}{\partial y} = N (x, y)$ .

$\frac{\partial f}{\partial y} = y + x^{3}$

Step 8

Find $\frac{\partial f}{\partial y}$ .

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Step 8.1

Differentiate $f$ with respect to $y$ .

$\frac{d}{d y} [y x^{3} + g (y)] = y + x^{3}$

Step 8.2

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

$\frac{d}{d y} [y x^{3}] + \frac{d}{d y} [g (y)] = y + x^{3}$

Step 8.3

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

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Step 8.3.1

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

$x^{3} \frac{d}{d y} [y] + \frac{d}{d y} [g (y)] = y + x^{3}$

Step 8.3.2

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

$x^{3} \cdot 1 + \frac{d}{d y} [g (y)] = y + x^{3}$

Step 8.3.3

Multiply $x^{3}$ by $1$ .

$x^{3} + \frac{d}{d y} [g (y)] = y + x^{3}$

Step 8.4

Differentiate using the function rule which states that the derivative of $g (y)$ is $\frac{d g}{d y}$ .

$x^{3} + \frac{d g}{d y} = y + x^{3}$

Step 8.5

Reorder terms.

$\frac{d g}{d y} + x^{3} = y + x^{3}$

Step 9

Solve for $\frac{d g}{d y}$ .

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Step 9.1

Move all terms not containing $\frac{d g}{d y}$ to the right side of the equation.

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Step 9.1.1

Subtract $x^{3}$ from both sides of the equation.

$\frac{d g}{d y} = y + x^{3} - x^{3}$

Step 9.1.2

Combine the opposite terms in $y + x^{3} - x^{3}$ .

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Step 9.1.2.1

Subtract $x^{3}$ from $x^{3}$ .

$\frac{d g}{d y} = y + 0$

Step 9.1.2.2

Add $y$ and $0$ .

$\frac{d g}{d y} = y$

Step 10

Find the antiderivative of $y$ to find $g (y)$ .

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Step 10.1

Integrate both sides of $\frac{d g}{d y} = y$ .

$\int \frac{d g}{d y} d y = \int y d y$

Step 10.2

Evaluate $\int \frac{d g}{d y} d y$ .

$g (y) = \int y d y$

Step 10.3

By the Power Rule, the integral of $y$ with respect to $y$ is $\frac{1}{2} y^{2}$ .

$g (y) = \frac{1}{2} y^{2} + C$

Step 11

Substitute for $g (y)$ in $f (x, y) = y x^{3} + g (y)$ .

$f (x, y) = y x^{3} + \frac{1}{2} y^{2} + C$

Step 12

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

$f (x, y) = y x^{3} + \frac{y^{2}}{2} + C$