Calculus Examples

$(2 x + 3 y - 7) d x + (3 x + 4 y - 10) d y = 0$

Step 1

Find $\frac{\partial M}{\partial y}$ where $M (x, y) = 2 x + 3 y - 7$ .

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Step 1.1

Differentiate $M$ with respect to $y$ .

$\frac{\partial M}{\partial y} = \frac{d}{d y} [2 x + 3 y - 7]$

Step 1.2

Differentiate.

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Step 1.2.1

By the Sum Rule, the derivative of $2 x + 3 y - 7$ with respect to $y$ is $\frac{d}{d y} [2 x] + \frac{d}{d y} [3 y] + \frac{d}{d y} [- 7]$ .

$\frac{\partial M}{\partial y} = \frac{d}{d y} [2 x] + \frac{d}{d y} [3 y] + \frac{d}{d y} [- 7]$

Step 1.2.2

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

$\frac{\partial M}{\partial y} = 0 + \frac{d}{d y} [3 y] + \frac{d}{d y} [- 7]$

Step 1.3

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

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Step 1.3.1

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

$\frac{\partial M}{\partial y} = 0 + 3 \frac{d}{d y} [y] + \frac{d}{d y} [- 7]$

Step 1.3.2

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

$\frac{\partial M}{\partial y} = 0 + 3 \cdot 1 + \frac{d}{d y} [- 7]$

Step 1.3.3

Multiply $3$ by $1$ .

$\frac{\partial M}{\partial y} = 0 + 3 + \frac{d}{d y} [- 7]$

Step 1.4

Since $- 7$ is constant with respect to $y$ , the derivative of $- 7$ with respect to $y$ is $0$ .

$\frac{\partial M}{\partial y} = 0 + 3 + 0$

Step 1.5

Combine terms.

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Step 1.5.1

Add $0$ and $3$ .

$\frac{\partial M}{\partial y} = 3 + 0$

Step 1.5.2

Add $3$ and $0$ .

$\frac{\partial M}{\partial y} = 3$

Step 2

Find $\frac{\partial N}{\partial x}$ where $N (x, y) = 3 x + 4 y - 10$ .

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Step 2.1

Differentiate $N$ with respect to $x$ .

$\frac{\partial N}{\partial x} = \frac{d}{d x} [3 x + 4 y - 10]$

Step 2.2

By the Sum Rule, the derivative of $3 x + 4 y - 10$ with respect to $x$ is $\frac{d}{d x} [3 x] + \frac{d}{d x} [4 y] + \frac{d}{d x} [- 10]$ .

$\frac{\partial N}{\partial x} = \frac{d}{d x} [3 x] + \frac{d}{d x} [4 y] + \frac{d}{d x} [- 10]$

Step 2.3

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

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Step 2.3.1

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

$\frac{\partial N}{\partial x} = 3 \frac{d}{d x} [x] + \frac{d}{d x} [4 y] + \frac{d}{d x} [- 10]$

Step 2.3.2

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

$\frac{\partial N}{\partial x} = 3 \cdot 1 + \frac{d}{d x} [4 y] + \frac{d}{d x} [- 10]$

Step 2.3.3

Multiply $3$ by $1$ .

$\frac{\partial N}{\partial x} = 3 + \frac{d}{d x} [4 y] + \frac{d}{d x} [- 10]$

Step 2.4

Differentiate using the Constant Rule.

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Step 2.4.1

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

$\frac{\partial N}{\partial x} = 3 + 0 + \frac{d}{d x} [- 10]$

Step 2.4.2

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

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

Step 2.5

Combine terms.

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Step 2.5.1

Add $3$ and $0$ .

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

Step 2.5.2

Add $3$ and $0$ .

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

Step 3

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

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Step 3.1

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

$3 = 3$

Step 3.2

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

$3 = 3$ is an identity.

Step 4

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

$f (x, y) = \int 2 x + 3 y - 7 d x$

Step 5

Integrate $M (x, y) = 2 x + 3 y - 7$ to find $f (x, y)$ .

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Step 5.1

Split the single integral into multiple integrals.

$f (x, y) = \int 2 x d x + \int 3 y d x + \int - 7 d x$

Step 5.2

Since $2$ is constant with respect to $x$ , move $2$ out of the integral.

$f (x, y) = 2 \int x d x + \int 3 y d x + \int - 7 d x$

Step 5.3

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

$f (x, y) = 2 (\frac{1}{2} x^{2} + C) + \int 3 y d x + \int - 7 d x$

Step 5.4

Apply the constant rule.

$f (x, y) = 2 (\frac{1}{2} x^{2} + C) + 3 y x + C + \int - 7 d x$

Step 5.5

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

$f (x, y) = 2 (\frac{x^{2}}{2} + C) + 3 y x + C + \int - 7 d x$

Step 5.6

Apply the constant rule.

$f (x, y) = 2 (\frac{x^{2}}{2} + C) + 3 y x + C - 7 x + C$

Step 5.7

Simplify.

$f (x, y) = x^{2} + 3 y x - 7 x + C$

Step 6

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

$f (x, y) = x^{2} + 3 y x - 7 x + g (y)$

Step 7

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

$\frac{\partial f}{\partial y} = 3 x + 4 y - 10$

Step 8

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

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Step 8.1

Differentiate $f$ with respect to $y$ .

$\frac{d}{d y} [x^{2} + 3 y x - 7 x + g (y)] = 3 x + 4 y - 10$

Step 8.2

Differentiate.

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Step 8.2.1

By the Sum Rule, the derivative of $x^{2} + 3 y x - 7 x + g (y)$ with respect to $y$ is $\frac{d}{d y} [x^{2}] + \frac{d}{d y} [3 y x] + \frac{d}{d y} [- 7 x] + \frac{d}{d y} [g (y)]$ .

$\frac{d}{d y} [x^{2}] + \frac{d}{d y} [3 y x] + \frac{d}{d y} [- 7 x] + \frac{d}{d y} [g (y)] = 3 x + 4 y - 10$

Step 8.2.2

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

$0 + \frac{d}{d y} [3 y x] + \frac{d}{d y} [- 7 x] + \frac{d}{d y} [g (y)] = 3 x + 4 y - 10$

Step 8.3

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

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Step 8.3.1

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

$0 + 3 x \frac{d}{d y} [y] + \frac{d}{d y} [- 7 x] + \frac{d}{d y} [g (y)] = 3 x + 4 y - 10$

Step 8.3.2

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

$0 + 3 x \cdot 1 + \frac{d}{d y} [- 7 x] + \frac{d}{d y} [g (y)] = 3 x + 4 y - 10$

Step 8.3.3

Multiply $3$ by $1$ .

$0 + 3 x + \frac{d}{d y} [- 7 x] + \frac{d}{d y} [g (y)] = 3 x + 4 y - 10$

Step 8.4

Since $- 7 x$ is constant with respect to $y$ , the derivative of $- 7 x$ with respect to $y$ is $0$ .

$0 + 3 x + 0 + \frac{d}{d y} [g (y)] = 3 x + 4 y - 10$

Step 8.5

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

$0 + 3 x + 0 + \frac{d g}{d y} = 3 x + 4 y - 10$

Step 8.6

Simplify.

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Step 8.6.1

Combine terms.

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Step 8.6.1.1

Add $0$ and $3 x$ .

$3 x + 0 + \frac{d g}{d y} = 3 x + 4 y - 10$

Step 8.6.1.2

Add $3 x$ and $0$ .

$3 x + \frac{d g}{d y} = 3 x + 4 y - 10$

Step 8.6.2

Reorder terms.

$\frac{d g}{d y} + 3 x = 3 x + 4 y - 10$

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 $3 x$ from both sides of the equation.

$\frac{d g}{d y} = 3 x + 4 y - 10 - 3 x$

Step 9.1.2

Combine the opposite terms in $3 x + 4 y - 10 - 3 x$ .

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Step 9.1.2.1

Subtract $3 x$ from $3 x$ .

$\frac{d g}{d y} = 4 y - 10 + 0$

Step 9.1.2.2

Add $4 y - 10$ and $0$ .

$\frac{d g}{d y} = 4 y - 10$

Step 10

Find the antiderivative of $4 y - 10$ to find $g (y)$ .

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Step 10.1

Integrate both sides of $\frac{d g}{d y} = 4 y - 10$ .

$\int \frac{d g}{d y} d y = \int 4 y - 10 d y$

Step 10.2

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

$g (y) = \int 4 y - 10 d y$

Step 10.3

Split the single integral into multiple integrals.

$g (y) = \int 4 y d y + \int - 10 d y$

Step 10.4

Since $4$ is constant with respect to $y$ , move $4$ out of the integral.

$g (y) = 4 \int y d y + \int - 10 d y$

Step 10.5

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

$g (y) = 4 (\frac{1}{2} y^{2} + C) + \int - 10 d y$

Step 10.6

Apply the constant rule.

$g (y) = 4 (\frac{1}{2} y^{2} + C) - 10 y + C$

Step 10.7

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

$g (y) = 4 (\frac{y^{2}}{2} + C) - 10 y + C$

Step 10.8

Simplify.

$g (y) = 2 y^{2} - 10 y + C$

Step 11

Substitute for $g (y)$ in $f (x, y) = x^{2} + 3 y x - 7 x + g (y)$ .

$f (x, y) = x^{2} + 3 y x - 7 x + 2 y^{2} - 10 y + C$