Doubt about neural network

So, i started getting interested in AI fair recently, and the last couple days I have been trying to develop an neural network to work with the MNIST dataset.
I was learning about the calculus and the algebra of it alland it was fairly easy untill the back propagation.
So, I started with random values on weights and biases, and it was about 8% accurate. I left it training for a bit and it jumped for something along 12%, I …

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