速算乘法 兩個位X兩個位

設：
*
=(10A+B)*(10C+D)
=(10A+B)*10C+(10A+B)*D
=[(10A+B)*10C]+[(10A+B)*D]
=[(10A)*10C+(B)*10C]+[(10A)*D+(B)*D]
=[(100AC)+(10BC)]+[(10AD)+(BD)]
=100AC+10BC+10AD+BD
=100*AC+10*(BC+AD)+BD
=100*AC+10*(AD+BC)+BD

step 1 . 上下相乘,,step 2 . 交叉相乘再加埋,之後補0

1.26x59
=2x5=10,,,9x6=54 =1054
=2x9=18,,,6x5=30 18+30=48,,補0,,=480
=1054+480
=1534


2.23x12
=2x1=2,,,3x2=6,,一位數要補0,,=06,,=206
=3x1=3,,,2x2=4,,,3+4=7,補0,,=70
=206+70=276

1.練好2 位 x 1 位速算 e.g. 16 x 7, 47 x 3 (要快記得永遠先乘十位後個位)

2. 練好加數速算 (同樣，先加百位，然後十位，最後個位)

例子:

498+531
= (400 + 500 ) + (90+ 30) + (8 +1)

= 900 + 120 + 9

= 1029