Question

Evaluate the line integral, where C is the given curve. (x + 4y) dx + x2 dy, C C consists of line segments from (0, 0) to (4, 1) and from (4, 1) to (5, 0)

Answer 1

Parameterize the line segments (call them $C_1$ and $C_2$, respectively, by

$\vec r_1(t)=(1-t)(0,0)+t(4,1)=(4t,t)$

$\vec r_2(t)=(1-t)(4,1)+t(5,0)=(4+t,1-t)$

both with $0\le t\le1$. Then

$\displaystyle\int_C(x+4y)\,\mathrm dx+x^2\,\mathrm dy$

$=\displaystyle\int_0^1\bigg((4t+4t)(4)+(4t)^2(1)\bigg)\,\mathrm dt+\int_0^1\bigg((4+t+4(1-t))(1)+(4+t)^2(-1)\bigg)\,\mathrm dt$

$=\displaystyle\int_0^115t^2+21t-8\,\mathrm dt=\boxed{\frac{15}2}$