Calculate the flux of the field F across the closed plane curve C. 17) F = xi + yj; the curve C is the counterclockwise path around the circle x2 + y2 = 16 18) F=y2i+xj; the curve C is the closed counterclockwise path formed from the semicircle r(t) = 4 cos ti + 4 sin tj, 0sts, and the straight line segment from (-4, 0) to (4, 0)