# what is the integral symbol called

## Approximating Space with Rectangles

How can we approximate the world between a operate and the x-axis if the curve is, effectively, curvy? We might use rectangles and triangles, however that may get actually awkward if the form is irregular. It seems to be extra helpful (and simpler) to easily use rectangles. The extra rectangles we use, the higher our approximation is.

## Riemann Sum

A Riemann sum for a operate f(x) over an interval [a, b] is a sum of areas of rectangles that approximates the world below the curve. Begin by dividing the interval [a, b] into n subintervals; every subinterval would be the base of 1 rectangle. We often make all of the rectangles the identical width: Δx. The peak of every rectangle comes from the operate evaluated in some unspecified time in the future in its subinterval. Then the Riemann sum is:

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[latex] f(x_1)Delta x + f(x_2)Delta x + f(x_3)Delta x + dots + f(x_n)Delta x [/latex]

## Sigma Notation

The upper-case Greek letter Sigma Σ is used to face for “add up the entire following issues.” Sigma notation is a strategy to compactly characterize a sum of many related phrases.

Utilizing the Sigma notation, the Riemann sum may be written [latex] sum_{i=1}^{n} (f(x_i)Delta x) [/latex].

That is learn aloud as “the sum from i equals 1 to n of f of x sub i delta x.”

## Definition of the Particular Integral

As a result of the world below a curve is so essential, it has a particular vocabulary and notation.

• The particular integral of a constructive operate f(x) from a to b is the world between f (on the prime), the x-axis (on the backside), and the vertical traces x = a (on the left) and x = b (on the best).

The shaded area is the world described by a particular integral.

## Notation for the Particular Integral

The particular integral of f from a to b is written [latex] int_a^b f(x)dx [/latex]

The ∫ image is known as an integral signal; it’s an elongated letter S, standing for sum. (The ∫ is definitely the Σ from the Riemann sum, written in Roman letters as an alternative of Greek letters.)

The dx on the tip have to be included; you’ll be able to consider∫ and dx as left and proper parentheses. The dx tells what the variable is—on this instance, the variable is x. (The dx is definitely the Δx from the Riemann sum, written in Roman letters as an alternative of Greek letters.)

The operate f is known as the integrand.

The a and b are known as the limits of integration.

## Utilizing the Idea in English

We combine, or discover the particular integral of a operate. This course of is known as integration.

## Signed Space

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Usually, space is at all times constructive. If the “peak” (from the operate) is a damaging quantity, then multiplying it by the width doesn’t give us precise space, it offers us the world with a damaging signal.

However now we’ll want the concept of damaging peak (and subsequently damaging areas). For instance, for those who’re mapping a cross-country journey, chances are you’ll discuss your peak above or under sea stage. Above sea stage, your peak could be a constructive quantity, and under sea stage, peak could be damaging.

## The Particular Integral and Signed Space

If the operate is above the x-axis all over the place between the bounds of integration, the world is constructive.

If the operate dips under the x-axis, the areas of the areas under the x-axis could have a damaging signal. These damaging areas take away from the particular integral.

[latex] int_{a}^{b} f(x)dx [/latex] = (Space above x-axis) – (Space under x-axis).

Unfavourable charges point out that an quantity is reducing. For instance, if f(t) is the speed of a automotive in a single course alongside a straight line at time t (miles/hour) , then damaging values of f point out that the automotive is touring in the other way: backwards. The particular integral of f is the change in place of the automotive in the course of the time interval. If the speed is constructive, constructive distance accumulates. If the speed is damaging, distance within the damaging course accumulates.

That is true of any price. For instance, if f(t) is the speed of inhabitants change (folks/yr) for a city, then damaging values of f would point out that the inhabitants of the city was getting smaller, and the particular integral could be the change within the inhabitants, a lower, in the course of the time interval.

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