I just want to introduce one more new notational convention.

Which is that when you're witting codes to implement backpropagation,

there will usually be some final output variable that you really care about.

So a final output variable that you really care about or that you want to optimize.

And in this case, this final output variable is J.

It's really the last node in your computation graph.

And so a lot of computations will be trying to compute the derivative of that

final output variable.

So d of this final output variable with respect to some other variable.

Then we just call that dvar.

So a lot of the computations you have will be to compute the derivative of the final

output variable, J in this case, with various intermediate variables,

such as a, b, c, u or v.

And when you implement this in software, what do you call this variable name?

One thing you could do is in Python,

you could give us a very long variable name like dFinalOurputVar/dvar.

But that's a very long variable name.

You could call this, I guess, dJdvar.

But because you're always taking derivatives with respect to dJ, with

respect to this final output variable, I'm going to introduce a new notation.

Where, in code, when you're computing this thing in the code you write,

we're just going to use the variable name dvar in order to represent that quantity.

So dvar in a code you write will represent the derivative of

the final output variable you care about such as J.

Well, sometimes, the last l with respect to the various intermediate quantities

you're computing in your code.

So this thing here in your code, you use dv to denote this value.

So dv would be equal to 3.

And your code, you represent this as da,

which is we also figured out to be equal to 3.

So we've done backpropagation partially through this computation graph.

Let's go through the rest of this example on the next slide.

So let's go to a cleaned up copy of the computation graph.

And just to recap, what we've done so

far is go backward here and figured out that dv = 3.

And again, the definition of dv, that's just a variable name,

where the code is really dJ/dv.

We've figured out that da = 3.

And again, da is the variable name in your code and that's really the value dJ/da.