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).

Great solution, I would just recommend you not to use -import and just use fully-qualified names for functions like first:square/1.

@elbrujohalcon I used import because I wanted to do it differently but you are absolutely right. Calling the square/1 function makes it look like the function is defined in the same module. Appending the module name with the function name makes it so clear :)

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 :)