（-1/42）/（1/6-2/7）+2/3+3/14
=（-1）/(7-12)+2/3+3/14(第一个分式上下同乘42)
=1/5+2/3+3/14
=(1*3+2*5)/15+3/14
=13/15+3/14
=(13*14+3*15)/15*14
=(182+45)/210
=227/210
227/210
= (-1/42)/(7/42-12/42) + 28/42+9/42
= -(-1/42)/(5/42) + 37/42
= 1/42*42/5 + 37/42
= 1/5 + 37/42
= 42/210+185/210
= 227/210
（-1/42）/（1/6-2/7）+2/3+3/14
=（-1/42)/(7/42-6*2/42)+2/3+3/14
=（-1/42)/(-5/42)+2/3+3/14
=1/5+2/3+3/14
=3/15+2*5/15+3/14
=13/15+3/14
=(13*14+3*15)/(15*14)
=227/210
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