Error math equation 3.5 Back propagation

At end of section 3.5 we are presented with the derivative of the cost function w.r.t. the weights of layer L.
Cost function is defined as C = 0.5(t-y)^2 where t: ‘output from NN’, y:‘target’

In the derivative we are given an equation containing a ‘2’. Can’t see how this value is supossed to be in the equation unless the cost function is C = (t-y)^2

Also; assuming that in layer L, t would be equal to a^L

We are later presented the derivative of the cost function w.r.t the weights of layer L-2. As the continuation of this equation seems to be the derivative w.r.t L-1 rather then L-2, there seems to be an error here.
(The text also talks about L-1 layer, not L-2 layer)

So if my math is wrong, I hope someone will take the time to explain how they arrived at these solutions.
If not are there some editing to for the devs

Hello @vemund.kallevik ,

The course creator has added some updates to section 3.5, as per your feedback. Please give it a look and let us know if it solves your doubts.