f(x)=√(ax²+bx+c)=√[a(x+b/2a)²+c-b²/4a]a>0且c-b²/4a≥0时 f(x)∈[√(c-b²/4a),+∞)a>0且c-b²/4a<0时 f(x)∈[0,+∞)a<0且c-b²/4a≥0时 f(x)∈[0,√(c-b²/4a)]
