（-1/7）÷1/42×5/6÷（-5/8）= (-1/7)*42 *(5/6) *(-8/5)= -6 *(5/6)*(-8/5)=8 （-1/2+2/3-1/4）×（-12）=(-6 +8 -3)/12 *(-12)= 1
