x²-ax+a+1≥0
(x-a/2)²≥a²/4-a-1
a²/4-a-1≤0
2-2√2≤a≤2+√2
∴当2-2√2≤a≤2+√2时,x∈R
当a<2-2√2,或a>2+√2时,x≤[a-√(a²-4a-4)]/2,或x≥[a+√(a²-4a-4)]/2