由已知可得:b=a+1,c=a+2,
则a 2+b 2-c 2-bc+5a
=a 2+(a+1) 2-(a+2) 2-(a+1)(a+2)+5a
=a2+a2+2a+1-a2-4a-4-a2-3a-2+5a
=-5