Calculus Examples

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

Step 1

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

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Differentiate $M$ with respect to $y$ .

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

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

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

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

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

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

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

$\frac{\partial M}{\partial y} = 3 x^{2} \cdot 1 + \frac{d}{d y} [y^{2}]$

Multiply $3$ by $1$ .

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

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

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

Step 2

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

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Differentiate $N$ with respect to $x$ .

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

Differentiate.

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

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

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

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

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

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

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

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

$\frac{\partial N}{\partial x} = 3 x^{2} + 2 y \cdot 1 + \frac{d}{d x} [3]$

Multiply $2$ by $1$ .

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

Differentiate using the Constant Rule.

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

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

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

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

Step 3

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

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

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

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

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

Step 4

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

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

Step 5

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

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Split the single integral into multiple integrals.

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

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

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

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

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

Apply the constant rule.

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

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

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

Simplify.

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

Step 6

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

$f (x, y) = y x^{3} + y^{2} x + g (y)$

Step 7

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

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

Step 8

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

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Differentiate $f$ with respect to $y$ .

$\frac{d}{d y} [y x^{3} + y^{2} x + g (y)] = x^{3} + 2 x y + 3$

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

$\frac{d}{d y} [y x^{3}] + \frac{d}{d y} [y^{2} x] + \frac{d}{d y} [g (y)] = x^{3} + 2 x y + 3$

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

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

$x^{3} \frac{d}{d y} [y] + \frac{d}{d y} [y^{2} x] + \frac{d}{d y} [g (y)] = x^{3} + 2 x y + 3$

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

$x^{3} \cdot 1 + \frac{d}{d y} [y^{2} x] + \frac{d}{d y} [g (y)] = x^{3} + 2 x y + 3$

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

$x^{3} + \frac{d}{d y} [y^{2} x] + \frac{d}{d y} [g (y)] = x^{3} + 2 x y + 3$

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

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

$x^{3} + x \frac{d}{d y} [y^{2}] + \frac{d}{d y} [g (y)] = x^{3} + 2 x y + 3$

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

$x^{3} + x (2 y) + \frac{d}{d y} [g (y)] = x^{3} + 2 x y + 3$

Move $2$ to the left of $x$ .

$x^{3} + 2 x y + \frac{d}{d y} [g (y)] = x^{3} + 2 x y + 3$

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

$x^{3} + 2 x y + \frac{d g}{d y} = x^{3} + 2 x y + 3$

Reorder terms.

$\frac{d g}{d y} + x^{3} + 2 x y = x^{3} + 2 x y + 3$

Step 9

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

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

$\frac{d g}{d y} = 3$

Step 10

Find the antiderivative of $3$ to find $g (y)$ .

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Integrate both sides of $\frac{d g}{d y} = 3$ .

$\int \frac{d g}{d y} d y = \int 3 d y$

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

$g (y) = \int 3 d y$

Apply the constant rule.

$g (y) = 3 y + C$

Step 11

Substitute for $g (y)$ in $f (x, y) = y x^{3} + y^{2} x + g (y)$ .

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

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