[填空题]
A motorboat whose speed 9rew2emngd4s zb42+m jk 9*aje :xb(t in still water i2h 7 f6uo*c. nam aumqb19xaome: .-bjs 2.60 $\mathrm{~m} / \mathrm{s}$ must aim upstream at an angle of 28.5$^{\circ}$ (with respect to a line perpendicular to the shore) in order to travel directly across the stream. (a) What is the speed of the current?
(b) What is the resultant speed of the boat with respect to the shore?