“△”是一元二次方程ax2+bx+c=0(a≠ 0)的根的判别式:△=b2-4ac．它的作用是不解方程即知一元二次方程根的情况．在二次三项式ax2+bx+c,一元二次不等式ax2+bx+c(?) 0,二次函数f(x)=ax2+bx+c中,也常见到它的身影．因此△在解题中有广泛的应用,但又常见同学们不能正确地应用它,有时该用不用,有时不该用却用了,有时使用不当．现举几例说明之． “△” is the discriminant of the root of the one-dimensional quadratic equation ax2 + bx + c = 0 (a ≠ 0): △ = b2-4ac. Its role is to understand the equation that is to know the root of the quadratic equation. In quadratic trinomial ax2+bx+c, a quadratic inequality ax2+bx+c(?) 0, and a quadratic function f(x)=ax2+bx+c, its presence is also commonly seen. Therefore, △ has a wide range of applications in solving problems, but it is often not used properly by students. It is sometimes used and not used, sometimes it is not used but sometimes it is used improperly. Here are a few examples.