first.erl

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

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

trebel(Num) ->
  mult(3,Num).

second.erl

-module(second).

-import('first', [square/1, mult/2]).

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


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

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

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

Wow! @elbrujohalcon and @sushant12, thank you for leaving these comments. I'm here to find how to use import from one module to another one. But now it really sounds more logic and clean to use filename:module() syntax instead of -import('blah', [blah/1]).
Nice :)