first.erl

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

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

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

second.erl

-module(second).
-export([hypotenuse/2, perimeter/2, area/2]).
% import functions from first.erl. I thought I can do this c(first)
% but it's erlang intercative shell only! I googled how to import a module
% and found this: http://erlang.org/doc/reference_manual/modules.html
-import(first, [multi/2, area/3]).

% gives the size of the hypotenuse of a right-angled triangle
% given the length of two other sizes.
hypotenuse(A,B) ->
  math:sqrt(first:square(A)+first:square(B)).

% gives an perimeter of right-angled triangle
% given the length of two short sides.
perimeter(A,B) ->
  A+B+hypotenuse(A,B).

% gives an area of right-angled triangle
% given the length of two short sides.
area(A,B) ->
 first: area(A,B,hypotenuse(A,B)).