Integration by parts is a trick for integrating many functions in the form f(x)g(x). It is a major part of integration at A Level.
Transcript
We’ve now seen how to fully master the one up, one down game…
For example, following the rules of the game, turn this into an integral that we can integrate [5xcos x dx]…
Differentiate this part, and integrate this one, to get this integral [5sin x dx]
And now that we’ve seen how to master that game, we’re on the verge of being able to integrate a whole bunch of more complex functions, like this one [example from above, 5xcos x dx]
Except, the only problem is, we definitely can’t just take an integral…and then differentiate one part and integrate the other to make it simpler…
As that would turn it into a totally different integral!
But fortunately, there is [click 2 times – wait for the animations] something we can do … [Pause audio for 2 seconds]
So, to integrate this function [same example still]
First, [click 2 times – waiting for the animations to finish] play the game [Pause audio for 2-3 seconds] to get this simpler integral [5sin x dx, see below for position]
Then, take the function you differentiated… [5x], and write it here…
Next, look at the function you integrated…
Take it’s integral, from here [sin x from new integral, not including 5 as that came from 5x]…
And write it here – so that it’s multiplied by this function…
Finally, place a minus symbol here…
And that’s it… this original integral, is identical to this… [must be laid out as below]
5xcos x dx ≡ 5xsin x -5sin x dx
And even though we’ll still have to integrate this to fully find this integral [LHS]…
This one’s much easier to integrate than this one!
And so next, playing the one up, one down game with this function [-9xsin x dx] gives us this integral [-9-cos x dx], which we can simplify [9cos x ]…but we’re not going to just yet… [revert to previous version]
So, if we want to rewrite this integral [first one, now lay screen out like below…]
-9x sin x dx≡_________-(-9)(-cos x )dx
Then what do we need to write here?
Here, we need to put the part we differentiated… [click 1 time] [-9x]
And then multiply it by the integral of the other part [Click 1 time] [-cos x from integral]. [Pause audio for 2 seconds]
Giving us this [(-9x)(-cos x )], and then this [9xcos x ]
And now we can rewrite this integral too [click 1 time] [last one to get rid of negatives and brackets]
So, this time, playing the one up, one down game with this function gives us this integral…
[6xsec2x dx 6tan x dx]
So, which of these is the correct way of rewriting this integral?
To rewrite this integral, we differentiate one part and integrate the other to get this integral…
And then, we need to take the term we differentiated…
Multiply it by the integral of the term we integrated…
And finally, subtract our new integral…
Giving us this expression
Now, rewriting integrals in this way is called integrating by parts…
Because it involves manipulating these two parts…
And so, we’ll practice integrating by parts in full….next…