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3: Integrals

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    34524
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    If we have a function \(f(x)\) which is well-defined for some \(a \le x \le b\), its integral over those two values is defined as \[\int_a^b dx\; f(x) \;=\; \lim_{N \rightarrow \infty} \, \sum_{n=0}^{N} \Delta x\; f(x_n) \;\;\;\mathrm{where}\;\; x_n = a + n\Delta x, \;\; \Delta x \equiv \left(\frac{b-a}{N}\right).\] This is called a definite integral, and represents the area under the graph of \(f(x)\) in the region between \(x=a\) and \(x=b\), as shown in the figure below. The function \(f(x)\) is called the integrand, and the two points \(a\) and \(b\) are called the bounds of the integral. The interval between the two bounds is divided into \(N\) segments, of length \((b-a)/N\) each. Each term in the sum represents the area of a rectangle, and as \(N\rightarrow \infty\), the sum converges to the area under the curve.

    clipboard_e7268f426693b00143c2821f372f5e056.png
    Figure \(\PageIndex{1}\)

    A multiple integral involves integration over more than one variable. For instance, when we have a function \(f(x_1,x_2)\) that depends on two independent variables, \(x_1\) and \(x_2\), we can perform a double integral by integrating over one variable first, then the other variable: \[\int_{a_1}^{b_1} dx_1 \int_{a_2}^{b_2} dx_2 \; f(x_1, x_2) \equiv \int_{a_1}^{b_1} dx_1 F(x_1)\quad\text{where}\;\;F(x_1) \equiv \int_{a_2}^{b_2} dx_2 \; f(x_1, x_2).\]


    3: Integrals is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Y. D. Chong via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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