y=1/2 +x-x²
=-(x²-x+1/4)+3/4
=-(x-1/2)²+3/4
平方项恒非负,(x-1/2)²≥0,因此-(x-1/2)²≤0 -(x-1/2)²+3/4≤3/4
y≤3/4,函数的值域为(-∞,3/4].