媒介変数曲線の微積分： 接線と面積

A parametric curve shows how a particle's position changes with a parameter. In parametric calculus, this means both x and y are defined as functions of a parameter, usually time. Analyzing the direction at each point means finding the slope of the tangent line.

The slope of the tangent line depends on how x and y change with respect to time. If y is a differentiable function of x, the chain rule gives the slope as the derivative of y with respect to t divided by the derivative of x with respect to t.

This ratio is defined when the derivative of x is not zero.

The curve has a horizontal tangent when the derivative of y is zero, and the derivative of x is nonzero.

On the other hand, it has a vertical tangent when the derivative of x is zero, and the derivative of y is nonzero.

For example, consider a curve traced by a moving ball with horizontal and vertical velocity components.

The slope of the tangent line equals the ratio of these components, which shows the tangent’s steepness and the ball’s direction of motion at each point. At the highest point on the curve, the vertical velocity is zero, showing a horizontal tangent.