Functional Programming in Erlang

first.erl

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

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

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

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

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

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

second.erl

-module(second).
-export([right_hypotenuse/2,right_perimeter/2,right_area/2]).

right_hypotenuse(A,B) ->
% The hypotenuse (C) of a right triangle comes from Pythagoras' theorem: C^2 = A^2 + B^2,
%   so C = sqrt (A^2 + B^2)
    math:sqrt(first:square(A)+first:square(B)).

right_perimeter(A,B) ->
% The perimeter of any triangle is the sum of the lengths three sides
    C=right_hypotenuse(A,B),
    A+B+C.
    
right_area(A,B) ->
    C=right_hypotenuse(A,B),
    first:area(A,B,C).

This is a perfect solution.
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