slopes and cross slopes. Calculus The concept of a slope is central to differential calculus. For non-linear functions, the rate of change varies along the curve. The derivative of the function at a point is the slope of the line tangent to the curve at the point, and is thus equal to the rate of change of the function at that point. If we let Δx and Δy be the distances (along the x and y axes, respectively) between two points on a curve, then the slope given by the above definition, , is the slope of a secant line to the curve. For a line, the secant between any two points is the line itself, but this is not the case for any other type of curve. For example, the slope of the secant intersecting y = x2 at (0,0) and (3,9) is 3. (The slope of the tangent at is also 3 − a consequence of the mean value theorem.) By moving the two points closer together so that Δy and Δx 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 Δy/Δx approaches as Δy and Δx 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 Δx approaches zero. Therefore, the slope of the tangent is the limit of Δy/Δx as Δx approaches zero, or dy/dx. We call this limit the derivative. Its value at a point on the function gives us the slope of the tangent at that point. For example, let y = x2. A point on this function is (−2,4). The derivative of this function is . So the slope of the line tangent to y at (−2,4) is . The equation of this tangent line is: or . Difference of slopes An extension of the idea of angle follows from the difference of slopes. Consider the shear mapping Then is mapped to . The slope of is zero and the slope of is . The shear mapping added a slope of . For two points on with slopes and , the image has slope increased by , but the difference of slopes