By moving the two points closer together so that Δ"y" and Δ"x" decrease, the secant line more closely approximates a tangent line to the curve, and as such the slope of the secant approaches that of the tangent. Using differential calculus, we can determine the limit, or the value that Δ"y"/Δ"x" approaches as Δ"y" and Δ"x" get closer to zero; it follows that this limit is the exact slope of the tangent. If "y" is dependent on "x", then it is sufficient to take the limit where only Δ"x" approaches zero. Therefore, the slope of the tangent is the limit of Δ"y"/Δ"x" as Δ"x" approaches zero, or "dy"/"dx". We call this limit the derivative.