10.已知在数列{an}中,a1=2,an+1=(-1)(an+2),n=1,2,3,....
(1)求{an}的通项公式;
(2)若数列{bn}中,b1=2,bn+1=,n=1,2,3,....证明:<bn≤a4n-3,n=1,2,3,....