原式
=1/42÷[1/2-1/3+5/7+4/9*(-6)]
=1/42÷[3/6-2/6+5/7-8/3]
=1/42÷[1/6+5/7-16/6]
=1/42÷[7/42+30/42-112/42]
=1/42÷(37-112)/42
=1/42×42/(-75)
=-1/75