uₙ=(n+1)/((2n-1)*3ⁿ⁻¹
u₁=(1+1) / ((2*1-1)*3¹⁻¹)=2 / (1*1)=2
u₂=(2+1) / ((2*2-1)*3²⁻¹)=3/(3*3)=1/3
u₃=(3+1) / ((2*3 - 1)*3³⁻¹)=4 / (5*9)=4/45
u₄=(4+1) / ((2*4 - 1)*3⁴⁻¹)=5 / (7*27)=5/189
u₅=(5+1) / ((2*5 - 1)*3⁵⁻¹)=6 / (9*81)=6/729.
