原式=lim(x->0) e^{[ln(2/π*arccosx)]/x}
=e^ {lim(x->0)[ln(2/π*arccosx)]/x}
=e^ {lim(x->0)[π/2arccosx*2/π*(-1/√(1-x^2))]}
=e^ {lim(x->0)[-arccosx/√(1-x^2)]}
=e^(-π/2)