Mistake in the backward propagation (Exercise 4, W1, A3)

Hello everyone, I hope you are doing well.
I have a problem in question 4 and I don’t know what I’m doing wrong.
I tried to read similar error posts and checked the most common mistakes, how to calculate theta_minus[i] in the same way as theta_plus[i] without paying attention to the sign.
However I keep getting the wrong answer.
Does anyone have any tips?

the output of my code is:
There is a mistake in the backward propagation! difference = 0.2850931567761624


I also have a question regarding index 2 of the norm. What does it mean? I thought it could be squared…


Hello @bruno_ramos_martins,

That error message is expected, and you will see the reasons if you continue to read the instructions followed by that message in the notebook. You will find the hint to get rid of that error!

As for the subscripted 2, it is read together with the || … || sign as the L2-norm. You can find more other names and some details of it here. You will use square to compute it. In general, we have the p-norm which replaces the subscripted 2 with scubscripted p, so the subscript has to be read along with the ||…|| sign and it has different meaning depending on the value of p.


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