|5a+b|与（a+5b+6）^2互为相反数
则|5a+b|+（a+5b+6）^2=0
|5a+b|=0,（a+5b+6）^2=0
5a+b=0
a+5b+6=0
两式相加得
6a+6b+6=0
a+b+1=0
a+b=-1