2又1/4+4又1/8+6又1/16+,...,+2n+1/2^(n+1）
=（2+4+6+……+2n)+[1/4+1/8+1/16+……+1/2^(n+1)]
=(2+2n)n/2+1/2-1/2^(n+1)
=n(n+1)+1/2-1/2^(n+1)