脱式计算下面各题. ①24×2425
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脱式计算下面各题.
①24×
(1)24×
24 25, =(25-1)× 24 25, =25× 24 25-1× 24 25, =24- 24 25, =23 1 25; (2)1994 1 2×79+ 6 25×790+31×7.9, =1994.5×79+2.4×79+3.1×79, =(1994.5+2.4+3.1)×79, =2000×79, =158000; (3)( 1 3- 1 2+1)×6, = 1 3×6- 1 2×6+1×6, =2-3+6, =5; (4)2007-2004+2001-1998+…+9-6+3, =2007-2004+2001-1998+…+9-6+3-0, =3×[(2007÷3-1)÷2+1], =3×[(669-1)÷2+1], =1005; |