Programming exercise for "FutureLearn/Programming in Earlang"

first.erl

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

mult(X, Y) ->
	X * Y.
double(X) ->
	mult(2,X).
treble(X) ->
	mult(3, X).
square(X) ->
	mult(X, X).
perimeter(A, B, C) ->
	A + B + C.
area(A,B,C) ->
	S = perimeter(A, B, C)/2,
	math:sqrt(S * (S - A) * (S - B) * (S - C)).

second.erl

-module(second).
-export([hypot/2, perimeter/2, area/2]).

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

perimeter(A, B) ->
	C = hypot(A, B),
	first:perimeter(A, B, C).

area(A, B) ->
	C = hypot(A, B),
	first:area(A, B, C).

Excellent!

Thank you @elbrujohalcon.