19 . 9 = (10 + 9 ) . 9
= 10 . 9 + 9 . 9
= 90 + 81 = 171
19 . 9
= 19 . 9 - 19 . 0
= 19 . ( 9 - 0 )
40 . 49
=40 . 49 - 40 . 0
= 40 . ( 49 - 0 )
~ Hok tốt ~
19.9=(20-1).(10-1)=20.(10-1)-1.(10-1)=200-20-10+1=171
40.49=40.(50-1)=2000-40=1960
\(19\cdot9\)
\(=19\cdot\left(10-1\right)\)
\(=19\cdot10-19\)
\(=190-19\)
\(=161\)
\(40\cdot49\)
\(=40\cdot\left(50-1\right)\)
\(=40\cdot50-40\)
\(=2000-40\)
\(=1960\)
