According to http://tutorial.math.lamar.edu/Classes/CalcIII/VectorArcLength.aspx i could use the formula int(sqrt x'^2+y'^2+z'^2, t , 0 , 2*pi) In other words , integral of the addition for the derivatives of x,y,z raised with the power of two.
But i get the following answer : "int((324*cos((9*t)/2)^2 + (729*sin((9*t)/2)^2)/4 + 81/(4*pi^2))^(1/2), t, 0, 2*pi)" that doesnt make sense. Shouldnt the t be calculated from 0 to 2pi?