n=1时,为奇数,a1=(1/2)^0=1;
n=2时,为偶数,a2=-(1/2)^1=-1/2;
n=3,a3=(1/2)^2=1/4;
n=4,a4=-(1/2)^3=-1/8
n=5,a5=(1/2)^4=1/16