原式=7 +2又5/6 +(-5又2/7) +2又1/6 +(-6又8/21)
=7 +(2又5/6 +2又1/6) +[ (-5又2/7) +(-6又8/21) ]
=7 +5 +[ -(5又2/7 +6又8/21) ]
=12 +(-11又2/3 )
=12 -11又2/3
=1/3