Solution: Solution: (1) Make the stem and leaf diagram as shown in the figure, and the median data in Yi Yi B group is 84.

(2) It is more appropriate to send a team for the following reasons:

∵.x A =85，。 x B =85，s？ 2 A =35.5，s？ 2 b =4 1，

∴.x A =。 x B，S？ 2 a < s？ 2 b，

∴ A's performance is relatively stable, so it is more appropriate to send A to participate in the competition.

(3) it is not established.

P(A)=68，P(B)=78，P (a)+P (b) = 138。

And 0≤p(A+B)≤ 1, so P(A)+P(B)=P(A+B) does not hold.