first.erl

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

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

%%mult(2, 3).

%%P = 10,12.

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

treble(A)->
    A*3.

square(A)->
    A*A.

second.erl

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

hypotenuse(A,B)->
    Asquare = first:square(A),
    Bsquare = first:square(B),
    math:sqrt(Asquare+Bsquare).

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

area(A,B)->
    A*B*0.5.