x+y=a
x-2y=5
两式相减3y=a-5<3
a<8
x+y=a乘以2得 2x+2y=2a与x-2y=5两式相加得
3x=2a+5>3
则a>-1
则-1<a<8
则a+1>0 a-8<0
则|a+1|+|a-8|=a+1+8-a=9