(2011•佛山一模)设n∈N+,圆Cn:x2+y2=R
n
x
(1)用xn表示Rn和an;
(2)若数列{xn}满足:xn+1=4xn+3,x1=3.
①求常数P的值使数列{an+1-p•an}成等比数列;
②比较an与2•3n的大小.