You are forgetting to apply the chain rule in the numerator...you could also multiply by x/x to get the same result of 0.
so for the the above it should be which would be 1*0/1=0
multiplying by x/x we would get which is equal is also equal to zero too but how? I feel like i'm missing some obvious step for version where we multiply by x/x
also is ? or is it just the last two that are the same?