First erlang program

first.erl

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

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

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

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

sq(X) ->
   mult(X,X).

treble(X) ->
   mult(X,3).

second.erl

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

hypotenuse(A,B) ->
   S = first:sq(A) + first:sq(B),
   math:sqrt(S).

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

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