What is the arc length of the curve given by r(t)= (9sqrt(2),e^(9t),e^(-9t))r(t)=(92,e9t,e9t) on t in [3,4]t[3,4]?

1 Answer
Mar 12, 2018

A zeroth-order approximation gives 2sinh(36)-2sinh(27)2sinh(36)2sinh(27) units.

Explanation:

r(t)=(9sqrt2, e^(9t), e^(−9t))r(t)=(92,e9t,e9t)

r'(t)=(0, 9e^(9t), -9e^(−9t))

Arc length is given by:

L=int_3^4sqrt(0+81e^(18t)+81e^(-18t))dt

Simplify:

L=9int_3^4sqrt(e^(18t)+e^(-18t))dt

Apply the identity coshx=1/2(e^x+e^-x):

L=9int_3^4sqrt(2cosh(18t))dt

Apply the identity cosh2x=2cosh^2x-1:

L=9sqrt2int_3^4sqrt(2cosh^2(9t)-1)dt

Factor out the larger piece:

L=18int_3^4cosh(9t)sqrt(1-1/2sech^2(9t))dt

For t in [3, 4], 1/2sech^2(9t)<1. Take the series expansion of the square root:

L=18int_3^4cosh(9t){sum_(n=0)^oo((1/2),(n))(-1/2sech^2(9t))^n}dt

Isolate the n=0 term:

L=18int_3^4cosh(9t)dt+18sum_(n=1)^oo((1/2),(n))(-1/2)^nint_3^4sech^(2n-1)(9t)dt

A zeroth-order approximation gives:

L~~2[sinh(9t)]_ 3^4

Hence:

L~~2sinh(36)-2sinh(27)