∑(1/3^n+1/5^n)
=(1/3)[1-(1/3)^n]/(1-1/3)+(1/5)[1-(1/5)^n]/(1-1/5)
=(1/2)*(1-1/3^n)+(1/4)*(1-1/5^n)
=3/4-(1/2)*(1/3^n)-(1/4)*(1/5^n)
当n→∞时
∑(1/3^n+1/5^n)=(1/3)/(1-1/3)+(1/5)/(1-1/5)
=1/2+1/4
=3/4
即有收敛性,收敛于3/4
求∑(1/3^n+1/5^n)的部分和,判断其敛散性,请写出解题过程.
∑(1/3^n+1/5^n)
=(1/3)[1-(1/3)^n]/(1-1/3)+(1/5)[1-(1/5)^n]/(1-1/5)
=(1/2)*(1-1/3^n)+(1/4)*(1-1/5^n)
=3/4-(1/2)*(1/3^n)-(1/4)*(1/5^n)
当n→∞时
∑(1/3^n+1/5^n)=(1/3)/(1-1/3)+(1/5)/(1-1/5)
=1/2+1/4
=3/4
即有收敛性,收敛于3/4