Didn't understand derivation of dL/dz and da/dz

[Musab_Bin_Gulfam]

In the above picture the derivation of dL/dz (dz) is a-y

My solution for dL/dz is below

Where did I go wrong? And why is da/dz is written as a(1-a) instead of e^-z/(1+e^-z)^2 ?

[paulinpaloalto]

Here’s a thread which covers all that in detail.

They refer you to google to get the actual derivative of sigmoid. Here’s a MathExchange thread that covers that.