# Line integral with respect to arc length

1. Jan 27, 2010

### Castilla

In a line integral with respect to arc length, we have something like f(x, y)ds "inside" the integral sign.

The ds tells us that we are working with the arc length function s, taking diferences (s_K+1 - s_k) in the sums that tend to the line integral.

Question: do we shall understand that x = x(s), y = y(s), or x and y are functions of another parameter? In this last case, this means that we work with two different parameters in the sums that tend to the integral?

2. Jan 27, 2010

### mathman

Ultimately x and y must be functions of s, although they be expressed in terms of another parameter.
Specifically ds2 = dx2 + dy2.

3. Jan 27, 2010

### Castilla

I think I have understood.

s = s(t), but s is an increasing function, so we have its inverse t = t(s). Then
x = x(t) = x(t(s)), y = y(t) = y(t(s)). Is this ok?

4. Jan 28, 2010

### HallsofIvy

If t is an arbitrary parameter, it does not necessarily follow that s is an increasing function of t.