(1) The probability of negative results of mixed blood of a group of K people is (1-p)k, and the probability of positive results is 1-( 1-p) k 。

x 1k+ 1 p( 1-p)k 1-( 1-p)k∴ex=k[ 1-( 1-p)k]+ 1？ ... (6 points)

(2) According to the meaning of the question, when f(k)= 1-0.9k+ 1k is less than 1 and the minimum value is taken, the best grouping method can be obtained.

∫f′(3)=-0.035 < 0，f′(4)= 0.006 > 0

And f (3) > f (4), ∴k=4 is the most suitable? ... (12 points)