How to calculate a easy function's derivative?

Stream: new members

[ShatteredLead]

I tried

import Mathlib
open Real
example:  deriv (fun x : ℝ => 3 * x - 4 * x ^ 3) = (fun x => 3 - 12 * x ^ 2) := by simp

But simp made no progress

[Kevin Buzzard]

Can you write your question as a #mwe ?

[Etienne Marion]

Here is a way:

import Mathlib

example : deriv (fun x : ℝ => 3 * x - 4 * x ^ 3) = (fun x => 3 - 12 * x ^ 2) := by
  ext x
  rw [deriv_sub, deriv_const_mul, deriv_id'', deriv_const_mul, deriv_pow]
  · ring
  all_goals fun_prop