6. Let a = [4, 7, 0], b= [3,-1, 5], c=[-6,2,0], and d = [1,-2, 8]. Find the angle between a and c. Between b and d. (10%)

7. Calculate this line integral by Stokes's theorem for the given F and C. Assume the Cartesian coordinates to be right-handed and the z-component of the surface normal to be nonnegative. F= [0,z3,0], C the boundary curve of the cylinder x2 + y2= 1, x≧ 0,y ≧ 0， 0 ≦z≦1 (10%)