CMake does not link library - c++

Question

My structure is actually very simple but make produces the following linker error:
/ld.exe:
CMakeFiles\hello.dir/objects.a(hello.cpp.obj):hello.cpp:(.text+0x2b):
undefined reference to `Mathe::sum(int, int)'
My folder structure is:
+ CMakeLists.txt
+ src
-- hello.cpp
+ lib
-+ mathe
--- mathe.h
--- mathe.cpp
--- CMakeLists.txt
CMakeLists.txt in lib/mathe folder looks like this:
cmake_minimum_required(VERSION 3.2)
project(mathe LANGUAGES CXX)
set(CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS TRUE)
add_library(mathe SHARED)
target_sources(mathe PRIVATE mathe.cpp)
the one in the root dir:
cmake_minimum_required(VERSION 3.8)
project(hello LANGUAGES CXX)
add_subdirectory(lib/mathe)
set(SOURCE src/hello.cpp)
add_executable(hello ${SOURCE})
target_link_directories(hello PRIVATE mathe)
target_include_directories(hello PRIVATE lib/mathe)
the hello.cpp has the following content:
#include <iostream>
#include "mathe.h"
using namespace std;
int main()
{
Mathe m;
int r = m.sum(2,3);
return 0;
}
the lib looks like this:
mathe.h:
#ifndef MATHE_H
#define MATHE_H
class Mathe
{
public:
int sum(int, int);
};
#endif
mathe.cpp:
#include "mathe.h"
int Mathe::sum(int a, int b)
{
return a+b;
}

Answer 1

replacing target_link_directories with target_link_libraries solved it.