unit workbook 4 calculus - wb 4.pdfletโs go on and see if we can find ๐ฆ๐ฆ โฒโฒ โฆ ๐ฆ๐ฆ...
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Unit WorkBook 4 โ Level 4 ENGโ U2 Engineering Maths ยฉ 2018 UniCourse Ltd. All Rights Reserved.
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Pearson BTEC Higher Nationals in Electrical and Electronic Engineering (RQF)
Unit 2: Engineering Maths (core)
Unit Workbook 4 in a series of 4 for this unit
Learning Outcome 4
Calculus
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3.1 Calculus There are a couple of great webpages for checking your answers to Calculus problemsโฆ
Check Differentiation answers
Check Integration answers
3.1.1 The Concept of the Limit, Continuity and the Derivative Calculus deals with functions which continually vary and is based upon the concept of a limit and continuity. Letโs refer to a diagram to understand the concept of a limitโฆ
Here we have indicated a point P on part of a function (the red curve) with Cartesian co-ordinates (๐ฅ๐ฅ1,๐ฆ๐ฆ1). We require another point Q on the function to be a small increment away from P and will designate a small increment with the symbol ๐ฟ๐ฟ (delta).
What we then have isโฆ
๐ท๐ท = (๐๐๐๐,๐๐๐๐)
๐ธ๐ธ = (๐๐๐๐ + ๐น๐น๐๐, ๐๐๐๐ + ๐น๐น๐๐)
Look now at the chord PQ (drawn as the straight line in blue). If we can determine the slope of this chord and then make it infinitesimally short we will end up with a tangent to the function. It is the slope of this tangent which forms the basis of Differential Calculus (normally called differentiation).
By inspection, we see that the slope of the chord is given byโฆ
๐๐๐๐๐๐๐๐๐๐ ๐๐๐๐ ๐๐๐๐๐๐๐๐๐๐ ๐ท๐ท๐ธ๐ธ =๐น๐น๐๐๐น๐น๐๐
If we deliberately make ๐ฟ๐ฟ๐ฅ๐ฅ approach zero (i.e. make it as short as possible) then we shall reach a limit, which may expressed mathematically asโฆ
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Worked Example 1
๐๐๐๐๐๐๐๐
= ๐น๐น๐๐๐๐๐๐๐๐๐๐๐๐๏ฟฝโฏโฏ๏ฟฝ ๐๐
๐น๐น๐๐๐น๐น๐๐
The term ๐๐๐ฆ๐ฆ ๐๐๐ฅ๐ฅโ is written in Leibnitz notation and indicates the slope of the chord when the chord only touches the function at one single point. This is achieved by continual reduction of ๐ฟ๐ฟ๐ฅ๐ฅ.
What we can now say is that we are able to find the slope of any function by adopting this process. Hence, given a function ๐๐(๐ฅ๐ฅ) we are able to differentiate that function, meaning find its slope at all points. We can writeโฆ
The slope at any point of a function ๐๐(๐ฅ๐ฅ) is given by ๐๐(๐๐(๐๐))๐๐๐๐
This process is called finding the derivative of a function.
3.1.2 Derivatives of Standard Functions What we donโt want to be doing is to spend too much time drawing graphs of functions just to work out the derivative. Fortunately there are standard ways to determine the derivative of functions and some of the frequent ones which engineers meet are given in the table below.
Function Derivative ๐ด๐ด๐ฅ๐ฅ๐๐ ๐๐๐ด๐ด๐ฅ๐ฅ๐๐โ1
A sin (๐ฅ๐ฅ) A cos (๐ฅ๐ฅ)
A cos (๐ฅ๐ฅ) โ๐ด๐ด sin (๐ฅ๐ฅ)
๐ด๐ด ๐๐๐๐๐๐ ๐๐๐ด๐ด๐๐๐๐๐๐
๐ด๐ด ๐๐๐๐๐๐๐๐(๐ฅ๐ฅ) ๐ด๐ด ๐ฅ๐ฅ๏ฟฝ
A sinh (๐ฅ๐ฅ) A cosh (๐ฅ๐ฅ)
A cosh (๐ฅ๐ฅ) A sinh (๐ฅ๐ฅ)
Letโs look at some examples of using these standard derivativesโฆ
Differentiate the function ๐๐ = ๐๐๐๐๐๐ with respect to ๐๐.
We see that this function is similar to that in row 1 of our table. We can see that ๐ด๐ด = 3 and ๐๐ = 4. We may then writeโฆ
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Worked Example 2
Worked Example 3
Worked Example 4
๐๐๐ฆ๐ฆ๐๐๐ฅ๐ฅ
= ๐๐๐ด๐ด๐ฅ๐ฅ๐๐โ1 = (4)(3)๐ฅ๐ฅ4โ1 = 12๐ฅ๐ฅ3
Differentiate the function ๐๐ = ๐๐๐๐โ๐๐ with respect to ๐๐.
We see that this function is similar to that in row 1 of our table. We can see that ๐ด๐ด = 6 and ๐๐ = โ5. We may then writeโฆ
๐๐๐ฆ๐ฆ๐๐๐ฅ๐ฅ
= ๐๐๐ด๐ด๐ฅ๐ฅ๐๐โ1 = (โ5)(6)๐ฅ๐ฅโ5โ1 = โ30๐ฅ๐ฅโ6
Differentiate the function ๐๐ = ๐๐๐๐ ๐๐๐๐๐๐(๐๐) with respect to ๐๐.
Donโt worry that ๐ฆ๐ฆ and ๐ฅ๐ฅ have disappeared here. We can use any letters we like. The letter ๐ฃ๐ฃ means voltage and ๐ก๐ก is time. All we want to do here then is to find ๐๐๐ฃ๐ฃ ๐๐๐ก๐ก๏ฟฝ .
We see that the function is similar to that in row 2 of our table. We may writeโฆ
๐๐๐ฃ๐ฃ๐๐๐ก๐ก
= ๐ด๐ด cos(๐ก๐ก) = 12 cos (๐ก๐ก)
Differentiate the function ๐๐ = ๐๐ ๐๐๐๐๐๐(๐๐) with respect to ๐๐.
The letter ๐๐ means current and ๐ก๐ก is time. All we want to do here then is to find ๐๐๐๐ ๐๐๐ก๐ก๏ฟฝ .
We see that the function is similar to that in row 3 of our table. We may writeโฆ
๐๐๐๐๐๐๐ก๐ก
= โ๐ด๐ด sin(๐ก๐ก) = โ5 sin (๐ก๐ก)
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Worked Example 5
Worked Example 6
Worked Example 7
Worked Example 8
Questions
Differentiate the function ๐๐ = ๐๐๐๐๐๐๐๐ with respect to ๐๐.
From row 4 of our table we can writeโฆ
๐๐๐ฆ๐ฆ๐๐๐ฅ๐ฅ
= ๐๐๐ด๐ด๐๐๐๐๐๐ = (2)(4)๐๐2๐๐ = 8๐๐2๐๐
Differentiate the function ๐๐ = ๐๐๐๐ ๐๐๐๐๐๐๐๐(๐๐) with respect to ๐๐.
We refer to row 5 of our table and writeโฆ
๐๐๐ฆ๐ฆ๐๐๐ฅ๐ฅ
=๐ด๐ด๐ฅ๐ฅ
=16๐ฅ๐ฅ
Differentiate the function ๐๐ = ๐๐ ๐๐๐๐๐๐๐๐(๐๐) with respect to ๐๐.
We refer to row 6 of our table and writeโฆ
๐๐๐ฆ๐ฆ๐๐๐ฅ๐ฅ
= ๐ด๐ด cosh(๐ฅ๐ฅ) = 9 cosh (๐ฅ๐ฅ)
Differentiate the function ๐๐ = โ๐๐๐๐.๐๐ ๐๐๐๐๐๐๐๐(๐๐) with respect to ๐๐.
We refer to row 7 of our table and writeโฆ
๐๐๐ฆ๐ฆ๐๐๐ฅ๐ฅ
= ๐ด๐ด sinh(๐ฅ๐ฅ) = โ14.5 sinh (๐ฅ๐ฅ)
Q3.1 Differentiate the function ๐๐ = ๐๐๐๐๐๐ with respect to ๐๐.
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Video
Q3.2 Differentiate the function ๐๐ = ๐๐๐๐โ๐๐ with respect to ๐๐.
Q3.3 Differentiate the function ๐๐ = ๐๐๐๐ ๐๐๐๐๐๐(๐๐) with respect to ๐๐.
Q3.4 Differentiate the function ๐๐ = ๐๐๐๐ ๐๐๐๐๐๐(๐๐) with respect to ๐๐.
Q3.5 Differentiate the function ๐๐ = ๐๐๐๐๐๐๐๐ with respect to ๐๐.
Q3.6 Differentiate the function ๐๐ = ๐๐๐๐ ๐๐๐๐๐๐๐๐(๐๐) with respect to ๐๐.
Q3.7 Differentiate the function ๐๐ = ๐๐๐๐ ๐๐๐๐๐๐๐๐(๐๐) with respect to ๐๐.
Q3.8 Differentiate the function ๐๐ = โ๐๐๐๐.๐๐๐๐ ๐๐๐๐๐๐๐๐(๐๐) with respect to ๐๐.
ANSWERS
These videos will boost your knowledge of differentiation
3.1.3 Notion of the Derivative and Rates of Change The notion of taking the derivative of a function and examining rate of change can be readily explained with reference to the diagram belowโฆ
Q3.1 40๐ฅ๐ฅ4
Q3.2 โ28๐ฅ๐ฅโ5
Q3.3 18 cos(๐ก๐ก)
Q3.4 โ11 sin(๐ก๐ก)
Q3.5 18๐๐3๐๐
Q3.6 20 ๐ฅ๐ฅโ
Q3.7 13 cosh(๐ฅ๐ฅ)
Q3.8 โ19.62 sinh(๐ฅ๐ฅ) Sample
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Challenge
Here we have plotted a section of a sine wave in red. Notice that we have used pink arrows to indicate the slope at various points on the sine wave. For example, at the start the sine wave is rising to a positive more rapidly than at any other point. The pink arrow indicates this positive growth by pointing upwards in the direction of the sine wave at that beginning point. The second arrow shows the slope of the sine wave at ๐๐ 2โ radians (or 900 if you like). Here it is at a plateau and has zero slope (meaning that it just moves horizontally, with no vertical movement at all). The third arrow indicates a negative slope (its moving down very steeply). The picture continues in a similar way for the remaining arrows.
Hereโs the really interesting bit. When we record all of the slopes on our sine wave we get the dotted waveform in blue. What does that look like? Of course, it is a cosine wave. So we have just proved graphically that finding the slope (differentiating) at all points on a sine wave leads to a cosine wave.
โด ๐๐{๐ฌ๐ฌ๐ฌ๐ฌ๐ฌ๐ฌ (๐๐)}
๐๐๐๐= ๐๐๐๐๐ฌ๐ฌ (๐๐)
โฆ as given in our table of standard derivatives in section 3.1.2.
If we had loads of time to kill we could prove all of the other derivatives graphically.
Determine a graphical proof (as above) forโฆ
๐๐{๐๐๐๐๐ฌ๐ฌ (๐๐)}๐๐๐๐
= โ๐ฌ๐ฌ๐ฌ๐ฌ๐ฌ๐ฌ (๐๐)
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3.2.3 Differentiation of Inverse Trigonometric Functions Suppose we have the equationโฆ
๐ฆ๐ฆ = ๐ด๐ด๐๐๐๐โ1(๐ฅ๐ฅ)
How do we then go about finding ๐๐๐ฆ๐ฆ ๐๐๐ฅ๐ฅโ ? Maybe we should write this in a different formโฆ
๐ฅ๐ฅ = sin(๐ฆ๐ฆ)
Now can differentiateโฆ
๐๐๐ฅ๐ฅ๐๐๐ฆ๐ฆ
= cos(๐ฆ๐ฆ) โด ๐๐๐๐๐๐๐๐ ๐ ๐ ๐๐๐ก๐กโ ๐ด๐ด๐๐๐๐๐๐๐ด๐ด: ๐๐๐ฆ๐ฆ๐๐๐ฅ๐ฅ
=1
cos (๐ฆ๐ฆ)
Next we shall remember one of our handy trig. Identitiesโฆ
๐๐๐๐๐ด๐ด2(๐ฆ๐ฆ) + ๐ด๐ด๐๐๐๐2(๐ฆ๐ฆ) = 1 โด ๐๐๐๐๐ด๐ด2(๐ฆ๐ฆ) = 1 โ ๐ด๐ด๐๐๐๐2(๐ฆ๐ฆ) โด cos(๐ฆ๐ฆ) = ๏ฟฝ1 โ ๐ด๐ด๐๐๐๐2(๐ฆ๐ฆ)
Since we know ๐ฅ๐ฅ = sin(๐ฆ๐ฆ) then if we square both sides we shall have ๐ฅ๐ฅ2 = ๐ด๐ด๐๐๐๐2(๐ฆ๐ฆ)
So we may re-write our transposed trig. identity asโฆ
cos(๐ฆ๐ฆ) = ๏ฟฝ1 โ ๐ฅ๐ฅ2
Now we can put this latest result back into our differentialโฆ
๐๐๐๐๐๐๐๐
=๐๐
๐๐๐๐๐ฌ๐ฌ (๐๐)=
๐๐โ๐๐ โ ๐๐๐๐
=๐๐
(๐๐ โ ๐๐๐๐)๐๐.๐๐ = (๐๐ โ ๐๐๐๐)โ๐๐.๐๐
That was a bit tricky, but we managed to find ๐ฆ๐ฆโฒ.
Letโs go on and see if we can find ๐ฆ๐ฆโฒโฒโฆ
๐ฆ๐ฆโฒ = (1 โ ๐ฅ๐ฅ2)โ0.5
This is where that shorthand notation for derivatives comes in really handyโฆ
๐ฟ๐ฟ๐๐๐ก๐ก ๐ข๐ข = 1 โ ๐ฅ๐ฅ2 โด ๐๐๐ข๐ข๐๐๐ฅ๐ฅ
= โ2๐ฅ๐ฅ
โด ๐ฆ๐ฆโฒ = ๐ข๐ขโ0.5
โด ๐๐๐ฆ๐ฆโฒ
๐๐๐ข๐ข= โ0.5๐ข๐ขโ1.5
โด ๐ฆ๐ฆโฒโฒ =๐๐๐ข๐ข๐๐๐ฅ๐ฅ
โ ๐๐๐ฆ๐ฆโฒ
๐๐๐ข๐ข= โ2๐ฅ๐ฅ ร โ0.5๐ข๐ขโ1.5
โด ๐๐โฒโฒ = ๐๐๐ ๐ โ๐๐.๐๐ = ๐๐(๐๐ โ ๐๐๐๐)โ๐๐.๐๐
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Question
Question
Q3.23 Find ๐๐โฒโฒ for ๐๐ = ๐๐๐๐๐๐โ๐๐(๐๐๐๐)
ANSWER
3.2.4 Differentiation of Inverse Hyperbolic Functions Consider the functionโฆ
๐ฆ๐ฆ = ๐ด๐ด๐๐๐๐โโ1(๐ฅ๐ฅ)
Our task here is to find ๐ฆ๐ฆโฒโฒ. We proceed in a similar manner to the previous problemโฆ
๐ฅ๐ฅ = sinh (๐ฆ๐ฆ)
โด ๐๐๐ฅ๐ฅ๐๐๐ฆ๐ฆ
= cosh(๐ฆ๐ฆ) โด ๐๐๐ฆ๐ฆ๐๐๐ฅ๐ฅ
=1
cosh (๐ฆ๐ฆ)
A useful identity isโฆ
๐๐๐๐๐ด๐ดโ2(๐ฆ๐ฆ)โ ๐ด๐ด๐๐๐๐โ2(๐ฆ๐ฆ) = 1 โด ๐๐๐๐๐ด๐ดโ2(๐ฆ๐ฆ) = 1 + ๐ด๐ด๐๐๐๐โ2(๐ฆ๐ฆ)
โด ๐๐๐๐๐ด๐ดโ2(๐ฆ๐ฆ) = 1 + ๐ฅ๐ฅ2 โด cosh(๐ฆ๐ฆ) = ๏ฟฝ1 + ๐ฅ๐ฅ2
โด ๐๐๐๐๐๐๐๐
= ๐๐โฒ =๐๐
โ๐๐ + ๐๐๐๐= (๐๐ + ๐๐๐๐)โ๐๐.๐๐
Now to find that second derivativeโฆ
๐ฆ๐ฆโฒ = (1 + ๐ฅ๐ฅ2)โ0.5
๐ฟ๐ฟ๐๐๐ก๐ก ๐ข๐ข = 1 + ๐ฅ๐ฅ2 โด ๐๐๐ข๐ข๐๐๐ฅ๐ฅ
= 2๐ฅ๐ฅ
โด ๐ฆ๐ฆโฒ = ๐ข๐ขโ0.5 โด ๐๐๐ฆ๐ฆโฒ
๐๐๐ข๐ข= โ0.5๐ข๐ขโ1.5
โด ๐ฆ๐ฆโฒโฒ =๐๐๐ข๐ข๐๐๐ฅ๐ฅ
โ ๐๐๐ฆ๐ฆโฒ
๐๐๐ข๐ข= 2๐ฅ๐ฅ ร โ0.5๐ข๐ขโ1.5 = โ๐ฅ๐ฅ(1 + ๐ฅ๐ฅ2)โ1.5
Q3.24 Find ๐๐โฒโฒ for ๐๐ = ๐๐๐๐๐๐๐๐โ๐๐(๐๐๐๐)
ANSWER
Q3.23 โ8๐๐(1โ4๐๐2)1.5
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Worked Example 22
3.3 Further Integration
3.3.1 Integration by Parts This technique provides us with a way to integrate the product of two simple functions. We shall develop a formula to use as a framework for the technique.
The starting point for our formula lies with the Product Rule, which you mastered in Section 3.1.5โฆ
๐๐๐๐๐ฅ๐ฅ
(๐ข๐ข๐ฃ๐ฃ) = ๐ข๐ข๐๐๐ฃ๐ฃ๐๐๐ฅ๐ฅ
+ ๐ฃ๐ฃ๐๐๐ข๐ข๐๐๐ฅ๐ฅ
Letโs rearrange this formulaโฆ
๐ข๐ข๐๐๐ฃ๐ฃ๐๐๐ฅ๐ฅ
=๐๐๐๐๐ฅ๐ฅ
(๐ข๐ข๐ฃ๐ฃ) โ ๐ฃ๐ฃ๐๐๐ข๐ข๐๐๐ฅ๐ฅ
If we now integrate both sides with respect to ๐ฅ๐ฅ then we need to place a โซ sign before each term and a ๐๐๐ฅ๐ฅ at the end of each term, like soโฆ
๏ฟฝ๐ข๐ข๐๐๐ฃ๐ฃ๐๐๐ฅ๐ฅ
๐๐๐ฅ๐ฅ = ๏ฟฝ๐๐๐๐๐ฅ๐ฅ
(๐ข๐ข๐ฃ๐ฃ)๐๐๐ฅ๐ฅ โ๏ฟฝ๐ฃ๐ฃ๐๐๐ข๐ข๐๐๐ฅ๐ฅ
๐๐๐ฅ๐ฅ
Some simplifications can be noticed here:
In the first integral the two ๐๐๐ฅ๐ฅ terms cancel each other out In the second integral we are taking the โintegral of the differentialโ of ๐ข๐ข๐ฃ๐ฃ. Since integration is the
reverse of differentiation then the โintegral of a differentialโ drops away, just leaving ๐ข๐ข๐ฃ๐ฃ In the third integral the two ๐๐๐ฅ๐ฅ terms cancel each other out
Performing these simplifications givesโฆ
๏ฟฝ๐ ๐ ๐๐๐๐ = ๐ ๐ ๐๐ โ๏ฟฝ๐๐ ๐๐๐ ๐
That is our formula for integration by parts.
What we do with the formula is to look at the left hand side and see the integral of a product. That product is composed of ๐ข๐ข and ๐๐๐ฃ๐ฃ. When using integration by parts you have a choice of which part of the product to call ๐ข๐ข and which to call ๐๐๐ฃ๐ฃ. Fortunately, your choice is guidedโฆ
The ๐ข๐ข part will become a constant after taking multiple derivatives The ๐๐๐ฃ๐ฃ part is readily integrated by using standard integrals.
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Worked Example 23
Determine: โซ๐๐ ๐ฌ๐ฌ๐ฌ๐ฌ๐ฌ๐ฌ(๐๐)๐๐๐๐
One part of our product is ๐ฅ๐ฅ and the other part is sin (๐ฅ๐ฅ). We need to make a choice as to which one to make ๐ข๐ข and which to make ๐๐๐ฃ๐ฃ. Our guidance tells us to make the ๐ฅ๐ฅ part equal to ๐ข๐ข since the derivative of ๐ฅ๐ฅ becomes 1 (i.e. a constant)โฆ
๐ฟ๐ฟ๐๐๐ก๐ก ๐ข๐ข = ๐ฅ๐ฅ
๐ฟ๐ฟ๐๐๐ก๐ก ๐๐๐ฃ๐ฃ = sin (๐ฅ๐ฅ)
We now try to form the terms in our integration by parts formulaโฆ
๐ผ๐ผ๐๐ ๐ข๐ข = ๐ฅ๐ฅ โด ๐๐๐ข๐ข๐๐๐ฅ๐ฅ
= 1 โด ๐๐๐ข๐ข = ๐๐๐ฅ๐ฅ
๐ผ๐ผ๐๐ ๐๐๐ฃ๐ฃ = sin(๐ฅ๐ฅ) โด ๐ฃ๐ฃ = โ cos (๐ฅ๐ฅ)
We can now writeโฆ
๏ฟฝ๐ ๐ ๐๐๐๐ = ๐ ๐ ๐๐ โ๏ฟฝ๐๐ ๐๐๐ ๐
๏ฟฝ๐ฅ๐ฅ sin(๐ฅ๐ฅ)๐๐๐ฅ๐ฅ = ๐ฅ๐ฅ(โ cos(๐ฅ๐ฅ)) โ๏ฟฝ(โ cos(๐ฅ๐ฅ))๐๐๐ฅ๐ฅ
โด ๏ฟฝ๐ฅ๐ฅ sin(๐ฅ๐ฅ)๐๐๐ฅ๐ฅ = โ๐ฅ๐ฅ cos(๐ฅ๐ฅ) + ๏ฟฝ cos(๐ฅ๐ฅ)๐๐๐ฅ๐ฅ
โด ๏ฟฝ๐๐๐ฌ๐ฌ๐ฌ๐ฌ๐ฌ๐ฌ(๐๐)๐๐๐๐ = โ๐๐๐๐๐๐๐ฌ๐ฌ(๐๐) + ๐ฌ๐ฌ๐ฌ๐ฌ๐ฌ๐ฌ(๐๐) + ๐ช๐ช
That was fairly straightforward. Always remember the constant of integration ๐ถ๐ถ at the end.
Letโs now try to integrate the product of an exponential and a cosine.
Determine: โซ๐๐๐๐๐๐ ๐๐๐๐๐ฌ๐ฌ(๐๐)๐๐๐๐
Our two products here are ๐๐2๐๐ and cos (๐ฅ๐ฅ). Neither of them will reduce to a constant when differentiated so it doesnโt matter which one we call ๐ข๐ข and which we call ๐๐๐ฃ๐ฃ. You will see here that we need to do integration by parts twice to arrive at our answer.
Just as in the previous worked example we need to decide on assignments for ๐ข๐ข and ๐๐๐ฃ๐ฃ, then find ๐ฃ๐ฃ and ๐๐๐ข๐ข. Letโs do thatโฆ
๐ฟ๐ฟ๐๐๐ก๐ก ๐ข๐ข = ๐๐2๐๐ โด ๐๐๐ข๐ข๐๐๐ฅ๐ฅ
= 2๐๐2๐๐ โด ๐๐๐ข๐ข = 2๐๐2๐๐๐๐๐ฅ๐ฅ
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Question
๐ฟ๐ฟ๐๐๐ก๐ก ๐๐๐ฃ๐ฃ = cos(๐ฅ๐ฅ) โด ๐ฃ๐ฃ = sin (๐ฅ๐ฅ)
To aid our working here (and save some ink) we normally assign the capital letter ๐ผ๐ผ to the integral in our question.
โด ๐ผ๐ผ = ๏ฟฝ๐๐2๐๐ cos(๐ฅ๐ฅ)๐๐๐ฅ๐ฅ
โด ๐ผ๐ผ = ๐๐2๐๐ sin(๐ฅ๐ฅ) โ๏ฟฝ sin(๐ฅ๐ฅ) 2๐๐2๐๐๐๐๐ฅ๐ฅ
This needs tidyingโฆ
โด ๐ผ๐ผ = ๐๐2๐๐ sin(๐ฅ๐ฅ) โ 2๏ฟฝ๐๐2๐๐sin(๐ฅ๐ฅ)๐๐๐ฅ๐ฅ
We were rather hoping for an answer, but what we seem to have is another โintegration by partsโ problem embedded into our development. Donโt worry, thatโs quite normal for this type of problem. All we do is attack that last part with โintegration by partsโ once more, using square brackets. Our desired result will then appearโฆ
๐ผ๐ผ = ๐๐2๐๐ sin(๐ฅ๐ฅ) โ 2 ๏ฟฝ๐๐2๐๐(โcos (๐ฅ๐ฅ)) โ๏ฟฝโcos (๐ฅ๐ฅ)2๐๐2๐๐๐๐๐ฅ๐ฅ๏ฟฝ
Expanding these terms givesโฆ
๐ผ๐ผ = ๐๐2๐๐ sin(๐ฅ๐ฅ) + 2๐๐2๐๐ cos(๐ฅ๐ฅ) โ 4๏ฟฝ๐๐2๐๐ cos(๐ฅ๐ฅ)๐๐๐ฅ๐ฅ
That last integral is the same as ๐ผ๐ผ, which is really handy, so we may now writeโฆ
๐ผ๐ผ = ๐๐2๐๐ sin(๐ฅ๐ฅ) + 2๐๐2๐๐ cos(๐ฅ๐ฅ) โ 4๐ผ๐ผ
โด 5๐ผ๐ผ = ๐๐2๐๐ sin(๐ฅ๐ฅ) + 2๐๐2๐๐cos (๐ฅ๐ฅ)
If we divide both sides by 5 then we have our final answerโฆ
๐ฐ๐ฐ =๐๐๐๐๐๐ ๐ฌ๐ฌ๐ฌ๐ฌ๐ฌ๐ฌ(๐๐) + ๐๐๐๐๐๐๐๐๐๐๐๐๐ฌ๐ฌ (๐๐)
๐๐
Q3.25 Determine: โซ๐๐๐๐๐๐๐๐๐๐๐๐(๐๐๐๐)๐๐๐๐
ANSWER
Q3.25 4๐๐4๐ฅ๐ฅ sin(2๐๐)โ2๐๐4๐ฅ๐ฅcos(2๐๐)
20
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3.4 Solution of Engineering Problems with Calculus
3.4.1 Charging RC Circuit For the circuit below, use Calculus to find the charge stored in the capacitor 2 seconds after the switch closes. Assume zero charge on the capacitor before this event.
For a capacitor charging via a DC source voltage, through a series resistor, the formula for instantaneous current isโฆ
๐๐ =๐ธ๐ธ๐ ๐ โ ๐๐๏ฟฝโ๐ก๐ก ๐ ๐ ๐ ๐ ๏ฟฝ ๏ฟฝ ๐ด๐ด๐ ๐ ๐๐๐ด๐ด
Since current is the defined as the rate of change of charge (๐ ๐ , carried by passing electrons) we may sayโฆ
๐๐ =๐๐๐ ๐ ๐๐๐ก๐ก
If we integrate both sides with respect to ๐ก๐ก we will haveโฆ
๏ฟฝ ๐๐ ๐๐๐ก๐ก = ๏ฟฝ๐๐๐ ๐ ๐๐๐ก๐ก
๐๐๐ก๐ก = ๏ฟฝ๐๐๐ ๐ = ๐ ๐
So we may say thatโฆ
๐ ๐ = ๏ฟฝ ๐๐ ๐๐๐ก๐ก
If we wish to find the amount of charge store stored within a certain time period (i.e. 2 seconds in this example) we writeโฆ
๐ ๐ (0โ2) = ๏ฟฝ ๐๐ ๐๐๐ก๐ก2
0
= ๏ฟฝ๐ธ๐ธ๐ ๐ โ ๐๐๏ฟฝโ๐ก๐ก ๐ ๐ ๐ ๐ ๏ฟฝ ๏ฟฝ
2
0
๐๐๐ก๐ก
=๐ธ๐ธ๐ ๐ ๏ฟฝ๐๐๏ฟฝโ1๐ ๐ ๐ ๐ ๏ฟฝ๐ก๐ก
โ1๐ ๐ ๐ถ๐ถ๏ฟฝ
๏ฟฝ
0
2
=โ๐ธ๐ธ๐ ๐ ๐ถ๐ถ๐ ๐
๏ฟฝ๐๐๏ฟฝโ๐ก๐ก ๐ ๐ ๐ ๐ ๏ฟฝ ๏ฟฝ๏ฟฝ0
2= โ๐ธ๐ธ๐ถ๐ถ ๏ฟฝ๐๐๏ฟฝโ๐ก๐ก ๐ ๐ ๐ ๐ ๏ฟฝ ๏ฟฝ๏ฟฝ
0
2
Since we know that ๐ ๐ = 1๐๐ฮฉ and ๐ถ๐ถ = 1๐๐๐น๐น then ๐ ๐ ๐ถ๐ถ = 1 ร 106 ร 1 ร 10โ6 = 106โ6 = 100 = 1 we may writeโฆ
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๐ ๐ (0โ2) = โ๐ธ๐ธ๐ถ๐ถ ๏ฟฝ๐๐๏ฟฝโ๐ก๐ก 1๏ฟฝ ๏ฟฝ๏ฟฝ0
2= โ๐ธ๐ธ๐ถ๐ถ[๐๐โ๐ก๐ก]02 = โ๐ธ๐ธ๐ถ๐ถ[๐๐โ2 โ ๐๐โ0] = โ๐ธ๐ธ๐ถ๐ถ[0.135 โ 1]
We also know that the DC supply voltage (๐ธ๐ธ) is 10Vโฆ
โด ๐๐(๐๐โ๐๐) = โ๐๐๐๐ ร ๐๐ ร ๐๐๐๐โ๐๐[โ๐๐.๐๐๐๐๐๐] = ๐๐.๐๐๐๐ ร ๐๐๐๐โ๐๐ ๐ช๐ช๐๐๐ ๐ ๐๐๐๐๐๐๐ช๐ช๐๐ = ๐๐.๐๐๐๐ ๐๐๐ช๐ช
The plot below illustrates the integration we have just performedโฆ
Current is on the vertical axis and time on the horizontal. When we multiply current by time we get charge, which is shown as the shaded area between 0 and 2 seconds. You may like to experiment with the Graph simulator to produce similar results.
3.4.2 Energising Inductor Consider the series RL circuit belowโฆ
If ๐ฌ๐ฌ = ๐๐๐๐ ๐ฝ๐ฝ, ๐น๐น = ๐๐๐๐๐๐ and ๐ณ๐ณ = ๐๐๐๐๐๐๐๐๐๐, use Calculus to determine the voltage across the inductor after ๐๐.๐๐๐๐๐๐. Use the following formulae in your solution developmentโฆ
๐๐ =๐ฌ๐ฌ๐น๐น๏ฟฝ๐๐ โ ๐๐โ๐น๐น๐๐ ๐ณ๐ณ๏ฟฝ ๏ฟฝ ๐๐๐๐๐๐ ๐๐๐ณ๐ณ = ๐ณ๐ณ
๐๐๐๐๐๐๐๐
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We start withโฆ
๐ฃ๐ฃ๐ฟ๐ฟ = ๐ฟ๐ฟ๐๐๐๐๐๐๐ก๐ก
= ๐ฟ๐ฟ๐๐ ๏ฟฝ๐ธ๐ธ๐ ๐ ๏ฟฝ1 โ ๐๐โ๐ ๐ ๐ก๐ก ๐ฟ๐ฟ๏ฟฝ ๏ฟฝ๏ฟฝ
๐๐๐ก๐ก
=๐ฟ๐ฟ๐ธ๐ธ๐ ๐ โ๐๐ ๏ฟฝ1 โ ๐๐โ๐ ๐ ๐ก๐ก ๐ฟ๐ฟ๏ฟฝ ๏ฟฝ
๐๐๐ก๐ก=๐ฟ๐ฟ๐ธ๐ธ๐ ๐ ๏ฟฝ๐ ๐ ๐ฟ๐ฟโ ๐๐โ๐ ๐ ๐ก๐ก ๐ฟ๐ฟ๏ฟฝ ๏ฟฝ = ๐ธ๐ธ๐๐โ๐ ๐ ๐ก๐ก ๐ฟ๐ฟ๏ฟฝ
Putting in the values for E, R, t and L givesโฆ
๐๐๐ณ๐ณ = ๐๐๐๐๐๐๏ฟฝโ๐๐๐๐
๐๐ร๐๐.๐๐ร๐๐๐๐โ๐๐๐๐.๐๐ ๏ฟฝ
= ๐๐.๐๐๐๐๐๐ ๐๐๐๐๐๐๐๐๐๐
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