an=(2n-1)/2^nsn=1/2+3/4+5/8+.+(2n-1)/2^n1/2sn=1/4+3/8+.+(2n-3)/2^n+(2n-1)/2^(n+1)sn-1/2sn=1/2+2/4+2/8+.+2/2^n-(2n-1)/2^(n+1)sn/2=1/2+2*(1/4+1/8+.+1/2^n)-(2n-1)/2^(n+1)sn/2=1/2+2*(1/2^2+1/2^3+.+1/2^n)-...