An instrument is a concrete or abstract tool intended for a purpose other than mechanical work, in particular a refined one.
Musical instruments are devices designed to produce music, frequently by plucking or striking a string or bell or drum surface, or by forcing air to resonate (see wind instruments).
... of each other. (In this case, the parabola does not intersect the x-axis at all.)
Note that when computing roots numerically, the usual form of the quadratic ...