27×39
=(30-3)×(40-1)
=1200-120-30+3
=1200-(120+30)+3
=1200-150+3
=1050+3
=1053
18×46
=(20-2)×(50-4)
=1000-100-80+8
=900-80+8
=820+8
=828
239×4
=(240-1)×4
=960-4
=956
8×126
=8×(120+6)
=960+48
=1008
23×6×7
=23×(6×7)
=23×42
=23×(40+2)
=920+46
=966
9×7×24
=63×24
=(60+3)×(20+4)
=1200+60+240+12
=1200+(60+240)+12
=1200+300+12
=1512
78×21-531
=(80-2)×21-531
=1680-42-531
=1680-(42+531)
=1680-573
=1680-500-70-3
=1180-70-3
=1110-3
=1107
列竖式算一下就行了，学习不能偷懒的！
1053.828.956.1008.966.1512.1107
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