# Why is the definite integral a single number?

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• mcastillo356

#### mcastillo356

Gold Member
TL;DR Summary
Been searching backwards in my textbook, but can't find it; I mean, the reason. I will provide a solved exercise
EXAMPLE 4 Find the area of the region ##R## lying above the line ##y=1## and below the curve ##y=5/(x^2+1)##.

Solution The region ##R## is shaded in Figure 5.24. To find the intersections of ##y=1## and ##y=5/(x^2+1)##, we must solve these equations simultaneously:

##1=\frac{5}{x^2+1}##

so ##x^2+1=5##,##x^2=4##, and ##x=\pm 2##

The area of the region ##R## is the area under the curve ##y=5/(x^2+1)## and above the ##x##-axis between ##x=-2## and ##x=2##, minus the area of a rectangle of width 4 and height 1. Since ##\tan^{-1}x## is an antiderivative of ##1/(x^2+1)##,

##A=\int_{-2}^2 \frac{5}{x^2+1}\ ,dx-4=2\int_0^2\frac{5}{x^2+1}\ ,dx-4=10\tan^{-1} x\left. \right |_0^2-4=10\tan{-1} 2-4## square units.

Observe the use of even symmetry (Theorem 3(h) of Section 5,4) to replace the lower limit of integration by 0. It si easier to susbstitute 0 into the antiderivative than -2.

THEOREM 3 Let ##f## and ##g## be integrable on an interval containing the points ##a##, ##b## and ##c##. Then

(...)

(h) The integral of an even function over an interval symmetric about zero is twice the integral over the positive half of the interval. If ##f## is an even function (i.e., ##f(-x)=f(x))##, then

##\int_{-a}^a f(x)\ ,dx=\int_0^a f(x)\ ,dx## Attempt: It is not explicited the unit

What is your question? The one in the title? And if so, why did you add the example?

If ##F(x)## is the anti-derivative of ##f(x)## then ##\displaystyle{\int_a^b\,f(x)\,dx=F(b)-F(a)}.## This is the fundamental theorem of calculus. The LHS is a definite integral and the RHS is a difference between two numbers, hence again a number.

• • mcastillo356 and bhobba
This is a consequence of the definition of the Darboux integral.

The upper and lower sums of a real-valued function with respect to a partition are real numbers by closure of addition and multiplication. The lower integral, as the supremum of the lower sums, is a real number by the least upper bound axiom; the upper integral, as the infimum of the upper sums, is a real number by the same axiom. A function is integrable if and only if these two numbers are equal, in which case the value of the integral is that number.

• mcastillo356
The area of the region ##R## is the area under the curve ##y=5/(x^2+1)## and above the ##x##-axis between ##x=-2## and ##x=2##, minus the area of a rectangle of width 4 and height 1. Since ##\tan^{-1}x## is an antiderivative of ##1/(x^2+1)##,

##A=\int_{-2}^2 \frac{5}{x^2+1}\ ,dx-4=2\int_0^2\frac{5}{x^2+1}\ ,dx-4=10\tan^{-1} x\left. \right |_0^2-4=10\tan{-1} 2-4## square units.

Not the answer to the question you asked, but a comment on your LaTeX. The very last number in your solution should be ##10\tan^{-1}2 - 4##. Omitting the '^' symbol makes what you wrote 10 tan -12 -4.

• mcastillo356
Hi, PF

Not the answer to the question you asked, but a comment on your LaTeX. The very last number in your solution should be ##10\tan^{-1}2 - 4##. Omitting the '^' symbol makes what you wrote 10 tan -12 -4.
Perfect.

What is your question? The one in the title?
Got it. I quote "Calculus, 7th ed, R. Adams & C. Essex"

(...) while the definite integral is a pure number, an area is a geometric quantity that impllicitly involves units. If the units along the ##x-## and ##y-##axes are, for example, metres, the area should be quoted in square metres (##m^2##). If units of length along the ##x-##axis and ##y-##axis are not specified, areas should be quoted in square units.

This is a consequence of the definition of the Darboux integral.

The upper and lower sums of a real-valued function with respect to a partition are real numbers by closure of addition and multiplication. The lower integral, as the supremum of the lower sums, is a real number by the least upper bound axiom; the upper integral, as the infimum of the upper sums, is a real number by the same axiom. A function is integrable if and only if these two numbers are equal, in which case the value of the integral is that number.
Thanks! A point of view of Darboux I haven't noticed untill now. Dense and precise.

Greetings!

Got it. I quote "Calculus, 7th ed, R. Adams & C. Essex"
(...) while the definite integral is a pure number, an area is a geometric quantity that implicitly involves units. If the units along the ##x##− and ##y##−axes are, for example, meters, the area should be quoted in square meters (##m^2##). If units of length along the##x##−axis and ##y##−axis are not specified, areas should be quoted in square units.

You can look at it as $$\int_a^b y\;dx=[y\cdot x]_{x=a}^{x=b}=(b-a)\cdot y=\underbrace{\int_{from\,[m]}^{to\,[m]}}_{\text{limits of width}} \underbrace{\,height\,[m]}_{=y} \cdot \underbrace{change \;in\;}_{=d} \underbrace{width\,[m]}_{=x} = area\,[m^2]$$
This is the simplest example and everything else are piecewise generalizations such that in the end, we get arbitrary integrals, up to integral curves (flows) through vector fields. It is a long way from width times height to meteorology!

Differentiation is accordingly. ##\dfrac{d\,xy}{dy}=x## and we can say "change of area divided by change of width is the height" (pretending the latter is constant as in the integral).

Hi, PF, @fresh_42

$$\int_a^b y\;dx=[y\cdot x]_{x=a}^{x=b}=(b-a)\cdot y=\underbrace{\int_{from\,[m]}^{to\,[m]}}_{\text{limits of width}} \underbrace{\,height\,[m]}_{=y} \cdot \underbrace{change \;in\;}_{=d} \underbrace{width\,[m]}_{=x} = area\,[m^2]$$

Is it possibly to see it entirely with algebra? Some subjects like dimensional analysis, differentiation, integration, are showed together. Very interesting.

Differentiation is accordingly. ##\dfrac{d\,xy}{dy}=x## and we can say "change of area divided by change of width is the height" (pretending the latter is constant as in the integral).

##\dfrac{d\,xy}{dy}=x## makes me wonder. Why did it takes to get infinitesimals?.

This is the simplest example and everything else are piecewise generalizations such that in the end, we get arbitrary integrals, up to integral curves (flows) through vector fields. It is a long way from width times height to meteorology!

I've browsed Google without success. Vector fields are related with meteorology, but how do I link it all together?

Is it possibly to see it entirely with algebra? Some subjects like dimensional analysis, differentiation, integration, are showed together. Very interesting.
It is an analytical concept and the definitions require limits. For a purely algebraic point of view, you would first have to establish such an environment so that the terms make sense. You can consider a differentiation as a derivation (no typo!) or a differential operator, but the corresponding integral operator uses an integral in its definition, i.e. again an analytical term.

##\dfrac{d\,xy}{dy}=x## makes me wonder. Why did it takes to get infinitesimals?
The infinitesimal notation - in this context - is more of a reminder not to forget that we deal with limits (change of) rather than having a meaning of their own.
$$\left. \dfrac{d}{d\,x}\right|_{x_0}f(x)=\left. \dfrac{d\,f}{d\,x}\right|_{x_0} =\displaystyle{\lim_{h \to 0}}\dfrac{f(x_0+h)-f(x_0)}{h}.$$
You cannot discuss them separately. Best you can get is Weierstrass's formula
$$f(x_0+h)=f(x_0)+ \underbrace{\left(\left. \dfrac{d}{d\,x}\right|_{x_0}f(x)\right)}_{=D_{x_0}f}\cdot h + r(h)$$
where all the infinitesimal quantities, all limits, anything going to zero, all those are put into the correction term ##r(h)## which is required to converge faster to zero than ##h## does, and ##D_{x_0}f## is merely a linear function and not the quotient of infinitesimals.

In other contexts, especially in physics, ##dx## can also mean a basis vector of the cotangent space or a differential, a Pfaffian form. These cases do not use quotients of ##d## expressions and can thus be explained without limits, or better: with hidden limits since we still use tangents.
I've browsed Google without success. Vector fields are related with meteorology, but how do I link it all together?
That was more of a rhetorical statement. Meteorology is full of (tangent) vector fields. You see at least one whenever you see a weather forecast, often literally with drawn vectors. Yet, the way from a bunch of secants converging to a tangent up to the theorem of the hedgehog as we call the hairy ball theorem is long, or to meteorology.

Here (pretty much at the beginning) is a list of 10 perspectives on the differentiation process:
https://www.physicsforums.com/insights/journey-manifold-su2mathbbc-part/
and I didn't even use the word slope.

You have to go this way if you study physics, at least the first steps to cotangent spaces and differential forms. If you want to study physics on a deeper level, then you will probably need pushforwards and pullbacks, too.

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• mcastillo356