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=3 ②810÷18 =810÷(9×2) =810÷9÷2 =90÷2 =45
③720÷18÷4 =720÷(18×4)=720÷72 =10
④630÷(21×2)=630÷21÷2 =30÷2 =15
三、加减凑整法 ①145+201 =145+200+1 =345+1 =346 ②234+98 =234+100-2 =334-2 =332