Question

Integrate f(x,y,z) = (x+y+z) / (x^2 +y^2 + z^2) over the path r(t) = 2ti +2tj +t k​, 0

Answer 1

The information about the path is incomplete, so I'll just use a domain of a ≤ t ≤ b, where a > 0 and b < ∞.

Differentiating r(t) gives

$\vec r(t) = 2t\,\vec\imath + 2t\,\vec\jmath + t\,\vec k \implies \dfrac{\mathrm d\vec r(t)}{\mathrm dt} = 2\,\vec\imath + 2\,\vec\jmath + \vec k$

with magnitude √(2² + 2² + 1²) = √5.

Then we have

$\displaystyle \int_C f(x,y,z)\,\mathrm ds = \int_a^b f(2t,2t,t) \left\|\frac{\mathrm d\vec r(t)}{\mathrm dt}\right\| \,\mathrm dt \\\\= \sqrt5 \int_a^b \frac{2t+2t+t}{(2t)^2+(2t)^2+t^2}\,\mathrm dt \\\\ = \sqrt5 \int_a^b \frac{5t}{5t^2} \,\mathrm dt \\\\ = \sqrt5 \int_a^b \frac{\mathrm dt}t \\\\ = \sqrt5 (\ln(b)-\ln(a)) = \boxed{\sqrt5 \ln\left(\frac ba\right)}$