-(19+6/7)*(20+1/7)
=-(139/7)*(141/7)
=-(139)*(141)/49
=-(140-1)*(140+1)/49
=-(140²-1)/49
=-(140²/49-1/49)
=-(20²-1/49)
=-399又48/49
或=-19599/49
-(19+6/7)*(20+1/7)
=-(139/7)*(141/7)
=-(139)*(141)/49
=-(140-1)*(140+1)/49
=-(140²-1)/49
=-(140²/49-1/49)
=-(20²-1/49)
=-399又48/49
或=-19599/49