What is the arc length of the curve given by r(t)= (9sqrt(2),e^(9t),e^(-9t))r(t)=(9√2,e9t,e−9t) on t in [3,4]t∈[3,4]?
1 Answer
A zeroth-order approximation gives
Explanation:
r(t)=(9sqrt2, e^(9t), e^(−9t))r(t)=(9√2,e9t,e−9t)
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
L=9int_3^4sqrt(2cosh(18t))dt
Apply the identity
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
L=18int_3^4cosh(9t){sum_(n=0)^oo((1/2),(n))(-1/2sech^2(9t))^n}dt
Isolate the
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)