loge[(1+x)1+x(1−x)1−x]
=loge(1+x)1+x+loge(1−x)1−x
=(1+x)loge(1+x)+(1−x)loge(1−x)
=[loge(1+x)+loge(1−x)]+x[log(1+x)−log(1−x)]
=loge(1−x2)+xloge(1+x1−x)
=(−x2−x42−x63−....)+2x(x+x33+x55+...)
=(−x2−x42−x63−...)+(2x2+2x33+2x55+....)
=(x2+x43.2+x65.3+...∞)
=2(x21.2+x43.4+x65.6+...∞)
⇒a=2