an = (-3)^0+ (-3)^1 +(-3)^2 +...+(-3)^(n-1) = (1/4)[1- (-3)^n]数列1,1-3,1-3+9,1-3+9-27,…前n项的和=a1+a2+...+an= (1/4) { n - (-3)[ 1- (-3)^n]/4 }=(1/4) [ n + (3/4)(1-(-3)^n ) ]=a1+a2+...+an= (1/4) { n - (-3)[ 1- (-3)^n]/4 }=(1/4) [ n + (3/4)(1-(-3)^n ) ] 看不太懂，能写详细点么