设
an=1-1/3+1/9-1/27 .(-1/3)^(n-1)
=1+(-1/3)+(-1/3)^2+(-1/3)^3+ .(-1/3)^(n-1)
=1*[1-(-1/3)^n]/[1-(-1/3)]
=[1-(-1/3)^n]/(4/3)
=3/4-(4/3)(-1/3)^n