求和1/2+3/4+5/8+.+(2n-1/2方)

求和1/2+3/4+5/8+.+(2n-1/2方)

--------------------------------------------------------------=1+[1-(1/2)^(n-1)]/(1-1/2)-(2n-1)/2^n
=1+2-1/2^(n-2))-(2n-1)/2^n
=3-(2n+3)/2^n