Approximate computation of the derivative number : nDeriv

### 4.3.2 Approximate computation of the derivative number : `nDeriv`

`nDeriv` takes as arguments : an expression `ex`, the variable name of this expression (by default `x`), and `h` (by default `h=0.001`).
`nDeriv(ex,x,h)` computes an approximated value of the derivative of the expression `ex` at the point `x` and returns :

`(f(x+h)-f(x+h))/2*h`

Input :

`nDeriv(x```^```2,x)`

Output :

`((x+0.001)```^```2-(x+-0.001)```^```2)*500.0`

Input :

`subst(nDeriv(x```^```2,x),x=1)`

Output :

`2`

Input :

`nDeriv(exp(x```^```2),x,0.00001)`

Output :

`(exp((x+1e-05)```^```2)-exp((x+-1e-05)```^```2))*50000`

Input :

`subst(exp(nDeriv(x```^```2),x,0.00001),x=1)`

Output :

`5.43656365783`

which is an approximate value of `2e=5.43656365692`.