脑算乘法第20讲
乘数首尾成倍,用乘数中小的乘被乘数,再前后移位加倍
38×63=38×3+2280 59×24=1180+236
=2394 =1416
82×42=164+3280 43×48=1720+344
=3444 =2064
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