Wednesday, December 2, 2015

[H2_Expository] Integration by Parts and the “d(etail)” Heuristic


Product Rule for Integration?
     How do we integrate a product of two functions e.g. find   ò x sin x dx ?   Unlike differentiation, there are not that many general rules (e.g. the Chain Rule, the Product Rule and the Quotient Rule) that we can use for integration.  However “Integration by parts” is similar to and can be obtained from the differentiation Product Rule


Applying Integration by Parts
     But how do we use the formula?  Before you do anything, analyse and classify the functions first.  You need to choose something for the  “u”  and  something for the “dv”  or  “dv/dx”.  For your choice of the “dv/dx” part, you can use the “d(etail)” heuristic as a guide.
     “e” is for exponential functions:      e.g. e2x, 3x.
     “t” is for trigonometric functions:   e.g. tan x, sin 3x, cos 2x.
     “a” is for algebraic functions:         e.g. 3x3, constants, 4xx2.
     “i” is for inverse functions:             e.g. sin-1 x, tann-1 5x.
     “l” is for logarithmic functions:      e.g. ln x, lg x, log2 x.
  
Here are some examples of choices for  “dv/dx”  and  “u” using the “d(etail)” heuristic.

Integral
Analysis
dv/dx
u
ò x sin x dx
x  is algebraic, sin x  is trigonometric,
t comes before a
sin x
x
ò e-x cos x dx
e-x  is exponential, cos x  is trigonometric,
e comes before t
e-x
cos x
ò x tan-1 x dx
x  is algebraic, tan-1 x  is inverse,
a comes before i
x
tan-1 x
ò ln x dx = ò (ln x)(1) dx
1  is algebraic, ln x  is logarithmic,
a comes before l
1
ln x

The above is actually equivalent to “liate” for the choice of  u, which is taught by many lecturers trained in American universities.  Once you have chosen  v,  the other part u  is automatically chosen,  and vice versa.  But personally, I think “d(etail)” is easier to remember: 
If you forget the details, just remember “d(etail)”!

I shall now illustrate the working of the first example with different styles of presentation.

Presentation 1 (for beginners – using “u” and “v” explicitly)


Presentation 2 (intermediate – using “pre-integration”)

With sufficient practice, the integration can be written down quickly as follows:-

Presentation 3 (advanced – for speed)
 
Remarks
     The “d(etail)” heuristic is a special one that is invented for Integration by Parts.  Like all heuristics, it is just a guideline or rule-of-thumb.  It works most of the time, but not all the time.  If you find that this does not work, you need to try different combinations of  u  and  dv.  The part chosen for “dv/dx” should be more easily integrable, or at least, you already know its integral.  After doing the by parts procedure, you should end up with an integral not more complicated than the original one.

Definite integrals
 

Suitable Levels
GCE ‘A’ Levels H2 Mathematics
* revision for IB Mathematics HL
* Advanced Placement (AP) Calculus BC
* other syllabuses that involve integration by parts
* any precocious or independent learner who loves calculus






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