a(x-5)/4≥x-4
ax-5a≥4x-16
(a-4)x≥-21
则
a>4,x≥-21/(a-4)
a=4,x取任意实数
a<4,x≤-21/(a-4)有些不对劲...对不起a(x-5)/4≥x-4，ax-5a≥4x-16，(a-4)x≥5a-16则a>4，x≥(5a-16)/(a-4)，，a=4，x取任意实数，，a<4，x≤(5a-16)/(a-4)