找规律：0,3,7,15,31,63,……第n个数是

设第n项为an.a1=0a2=3=2^0+2^1a3=7=3+4=2^0+2^1+2^2a4=15=7+8=2^0+2^1+2^2+2^3a5=31=15+16=2^0+2^1+2^2+2^3+2^4a6=63=31+32=31+2^5=2^0+2^1+2^2+2^3+2^4+2^5…………an=2^0+2^1+2^2+2^3+2^4+...+2^(n-1)=(2^n-1)/(2-...