>>345
敗in(nx)/3^n
=im 納n=0,∞]exp(inx)/3^n
=im 1/(1-(expix)/3)
=im (1-(exp(-ix)/3))/((1-(expix/3))(1-(exp(-ix)/3)))
=im (1-(exp(-ix)/3))/(1-(2/3)re expix + 1/9)
=im (1-(exp(-ix)/3))/(10/9-(2/3)cosx)
=sinx/(10/3-2cosx)