5a²＋[a²＋（5a²-2a）-2(a2-3a)]
=5a²+a²+5a²-2a-2a²+6a
=9a²+4a
当a=-1/2时
原式=9×(-1/2)²+4×(-1/2)
=9/4-2
=1/4