lim (x->pai/2) (lnsinx)/(pai-2x)^2 (0/0)
=(L')=lim(x->pai/2)[cosx/sinx]/(8x-4pai)(0/0)
=(L')=lim(x->pai/2)-sinx/[cosx(8x-4pai)+8sinx] (代入)
=-1/8