This video demonstrates the first reverse chain rule ‘trick’, which tells you how to integrate functions in the form f'(x) divided by f(x).
Transcript
We’ve now seen that, before we can use the first integration trick, we sometimes need to check whether the top of a fraction… is a multiple of the bottom’s derivative
And last time, we saw how to do that!
Meaning that, finally, we can put it all together and integrate these fractions…
So, given this integral… [new integral, 14x+213×2+9xdx]
Before we can use the first integration trick, we need to check whether this [top] is a multiple of the bottom’s derivative…
And to do that, we first differentiate the bottom expression, which, in this case, gives…
In this case, the derivative of this expression is this [6x+9]
Next, we take this derivative [to the side], put it in brackets, and give it a coefficient of…
The coefficient is just this number [14], divided by this number [6] [1466x+9]
Which we could then simplify to 73.
Next, expanding these brackets tells us that…
Expanding these brackets gives us this…which is the same as this…meaning that this expression is a multiple of this derivative! [click one time]
So, we’ve confirmed that we can use the first integration trick to integrate this fraction…
And now, since we’ve already shown that this [14x+21]…
Is the same as this [73(6x+9)]
We can just rewrite our fraction like this! [73(6x+9)3×2+9xdx]
And finally, by taking this coefficient outside of the integral [736x+93×2+9xdx]…we can integrate…
We can integrate to get this [73ln 3×2+9x +c]
That’s all there is to it!
So, to sum up, in the process of checking whether the top of a fraction is a multiple of the bottom’s derivative, we’ll get an expression like this [73(6x+9)]…with the derivative, multiplied by some coefficient…
And then, if the top of the fraction does turn out to be a multiple of the derivative…we can just replace it with this expression [click one time]…take the coefficient outside the integral…and integrate!