Given,
f(x)=4∣2x+3∣+9[x+21]−12[x+20]
x∈(−20,20)
Now we know that [x] is non-differentiable when x∈I
So, 12[x+20] is not differentiable at x=I∈−19,−18,…0,…19=39
Now 9[x+21] is not differentiable when x=I+21
So, here also total non-differentiable points are 39
Now checking modulus function
We know that modulus function are non-differentiable at point where modulus function is zero,
So at x=2−3 modulus function is zero
Also f(x) at x=2−3 is not continuous and non-differentiable,
So, number of point of non-differentiability=39+39+1=79