Length of an arc : arcLen

### 2.17.2 Length of an arc : `arcLen`

`arcLen` takes four arguments : an expression ex (resp a list of two expressions [ex1,ex2]), the name of a parameter and two values a and b of this parameter.
`arcLen` computes the length of the curve define by the equation y=f(x)=ex (resp by x=ex1,y=ex2) when the parameter values varies from a to b, using the formula `arcLen(f(x),x,a,b)=`
`integrate(sqrt(diff(f(x),x)```^```2+1),x,a,b)`
or
`integrate(sqrt(diff(x(t),t)```^```2+diff(y(t),t)```^```2),t,a,b)`.
Examples

- Compute the length of the parabola y=x2 from x=0 to x=1.
  Input :
  `arcLen(x```^```2,x,0,1)`
  or
  `arcLen([t,t```^```2],t,0,1)`
  Output :
  `-1/4*log(sqrt(5)-2)-(-(sqrt(5)))/2`
- Compute the length of the curve y=cosh(x) from x=0 to x=ln(2).
  Input :
  `arcLen(cosh(x),x,0,log(2))`
  Output :
  `3/4`
- Compute the length of the circle x=cos(t),y=sin(t) from t=0 to t=2*π.
  Input :
  `arcLen([cos(t),sin(t)],t,0,2*pi)`
  Output :
  `2*pi`