variable {R : Type _} [Ring R]

   add_assoc    : ∀ a b c : R, (a + b) + c = a + (b + c)
   add_comm     : ∀ a b : R,   a + b = b + a
   zero_add     : ∀ a : R,     0 + a = a
   add_left_neg : ∀ a : R,     -a + a = 0
   mul_assoc    : ∀ a b c : R, a * b * c = a * (b * c)
   mul_one      : ∀ a : R,     a * 1 = a
   one_mul      : ∀ a : R,     1 * a = a
   mul_add      : ∀ a b c : R, a * (b + c) = a * b + a * c
   add_mul      : ∀ a b c : R, (a + b) * c = a * c + b * c

import Mathlib.Algebra.Ring.Defs

variable {R : Type _} [Ring R]
variable (a : R)

example : a + 0 = a :=
sorry

import Mathlib.Data.Real.Basic
import Mathlib.Tactic

variable (a b c : ℝ)

example
  (h : a + b = c)
  : (a + b) * (a + b) = a * c + b * c :=
sorry

import Mathlib.Data.Real.Basic
import Mathlib.Tactic

variable (a b c d : ℝ)

example
  (h1 : c = d * a + b)
  (h2 : b = a * d)
  : c = 2 * a * d :=
sorry

import Mathlib.Data.Real.Basic
import Mathlib.Tactic

variable (a b : ℝ)

example : (a + b) * (a - b) = a^2 - b^2 :=
sorry

import Mathlib.Data.Real.Basic
import Mathlib.Tactic

variable (a b c d : ℝ)

example
  : (a + b) * (c + d) = a * c + a * d + b * c + b * d :=
sorry

import Mathlib.Data.Real.Basic
import Mathlib.Tactic

variable (a b c : ℝ)

example :
  (a + b) * (a + b) = a * a + 2 * (a * b) + b * b :=
sorry

import Mathlib.Data.Real.Basic
import Mathlib.Tactic

example
  (a b c d : ℝ)
  (h1 : c = b * a - d)
  (h2 : d = a * b)
  : c = 0 :=
by sorry

import Mathlib.Data.Real.Basic
import Mathlib.Tactic

example
  (a b c d e f : ℝ)
  (h : b * c = e * f)
  : ((a * b) * c) * d = ((a * e) * f) * d :=
by sorry

import Mathlib.Tactic
import Mathlib.Data.Real.Basic

example
  (a b c d e f : ℝ)
  (h1 : a * b = c * d)
  (h2 : e = f)
  : a * (b * e) = c * (d * f) :=
by sorry

import Mathlib.Tactic
import Mathlib.Data.Real.Basic

example
  (a b c : ℝ) : a * (b * c) = b * (a * c) :=
by sorry