5a方-[a方+（5a方-2a）-2（a方-3a）]
=5a²-【a²+5a²-2a-2a²+6a]
=5a²--a²-5a²+2a+2a²-6a
=a²-4a
=1/4-4*1/2
=1/4-2
=-1又3/4