Let f(x) = |x|.
Visualize the graph of this function.
The derivative of f(x) when x < 0 is ??????.
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Expand the function f(x)=lxl
you will get that f(x)= x when a is greater than or equal to 0 and f(x)= -x when x is less than 0.
It should be clear on what to do from here.
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