# Possible error in lecture slides

DLS Course 1: Lecture Notes - In slide 33 of C1_w2.pdf (https://community.deeplearning.ai/uploads/short-url/zWmJABeU2UIO9iripYpx3VPRqpV.pdf) - dz(i) is a(i)* (1 - a(i)) while it should be (a(i) - y(i)). This seems to be corrected in the videos.

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Hi Sriram_Kollipara,

Thanks for your post, which will help current and future learnings! For anyone reading this post, do note that the lecture notes are not updated, and that you are encouraged to make your own notes.

Hello Sriram.
I too noticed this error and wanted to confirm your post.

I also struggled through this, but had to work out the math.

â€śdzâ€ť = a-y

dL/da. = -y/a+(1-y)/(1-a)
da/dz = a(1-a)

You can workout above with the later two equations using the chain rule.
â€śdzâ€ť = dL/dz = dL/da*da/dz

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As a former teacher I just canâ€™t get over how astonishingly lazy it is to simply declare that the lecture notes are just â€śnot updatedâ€ť.

This course has literally eight hundred thousand enrolments on Coursera as of November 2023. If a typo in the formulas costs just one second of being confused on average per viewer, thatâ€™s more than nine days of wasted human life in aggregate.

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