∵abc0
∴a,b,c两正一负
不妨设c0,b>0
∴x=|a|/a+|b|/b+|c|/c
=1+1-1
=1
∴多项式ax的四次方+bx的二次方+c-5
=a*1^4+b*1^2+c-5
=a+b+c-5
=2-5
=-3