回家作业有理数的减法4.9-(-8.1)+(-8)-(+7.8)
①负负得正;
②正负得负;
③负正得负;
4.9-(-8.1)+(-8)-(+7.8)
=4.9+8.1-8-7.8
=(4.9+8.1)-(8+7.8)
=13-15.8
=-(15.8-13)
=-2.8
负负得正,正负得负,负正得负
解;4.9-(-8.1)+(-8)-(+7.8)
=4.9+8.1-8-7.8
=4.9+(8.1-8)-7.8
=4.9+0.1-7.8
=5-7.8
=-2.8
4.9-(-8.1)+(-8)-(+7.8)
=4.9+8.1-8-7.8
=-2.8
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