In your project that uses the library make the LIBS variable point to your lib's path.
To include files from the library, add the library folder to the INCLUDEPATH and then do a regular #include in your code files.
e.g:
# the binary's .pro
LIBS += c:/mylibs/math.lib
INCLUDEPATH += c:/mylibs
Edited:
-L tells qmake that the path is a directory that it can search for libraries
-l tells it that the path is a file, but take note of the observation below.
From the qmake docs:
This variable contains a list of
libraries to be linked into the
project. You can use the Unix -l
(library) and -L (library path) flags
and qmake will do the correct thing
with these libraries on Windows
(namely this means passing the full
path of the library to the linker).
The only limitation to this is the
library must exist, for qmake to find
which directory a -l lib lives in.
Note: On Windows, specifying libraries
with the -l option, as in the above
example, will cause the library with
the highest version number to be used;
for example, libmath2.lib could
potentially be used instead of
libmathlib. To avoid this ambiguity,
we recommend that you explicitly
specify the library to be used by
including the .lib file name suffix.