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第1个回答 2019-04-30
21x57+ 42x21
=21x(57+42)
=21x99
=21x(100-1)
=21x100-21
=2100-21
=2079本回答被网友采纳
=21x(57+42)
=21x99
=21x(100-1)
=21x100-21
=2100-21
=2079本回答被网友采纳
第2个回答 2019-04-30
21×57
=(20+1)×(60-3)
=20×60-20×3+1×60-1×3
=1200-60+60-3
=1197
42×21
=(40+2)×(20+1)
=40×20+40×1+20×2+2×1
=800+40+40+2
=882
=(20+1)×(60-3)
=20×60-20×3+1×60-1×3
=1200-60+60-3
=1197
42×21
=(40+2)×(20+1)
=40×20+40×1+20×2+2×1
=800+40+40+2
=882
第3个回答 2019-04-30
21X57+42X21
=21X(57+42)
=21X99
=21X100-21X1
=2100-21
=2079
=21X(57+42)
=21X99
=21X100-21X1
=2100-21
=2079