My first Erlang program

first.erl

-module(first).
-export([double/1,mult/2,area/3,square/1]).

mult(X,Y) ->
  X*Y.

double(X) ->
  mult(2,X).

square(A) ->
  mult(A,A).

area(A,B,C) ->
  S = (A+B+C)/2,
  math:sqrt(S*(S-A)*(S-B)*(S-C)).

second.erl

-module(second).
-import(first, [square/1, mult/2]).
-export([hypotenuse/2,perimeter/2,area/2]).

hypotenuse(A,B) ->
  math:sqrt(square(A)+square(B)).

perimeter(A,B) ->
  A+B+hypotenuse(A,B).

area(A,B) ->
  mult(A,B)/2.