a, 53 x 39 + 47 x 39 - 53 x 21 - 47 x 21
= 53 x (39-21) + 47 x (93-21)
=53 x 18 + 47 x 18
= (53 +47) x 18
= 100 x18
=1800
b,2 x 53 x 12 + 4 x 6 x 87 - 3 x 8 x 40
=24 x 53 + 24 x 87 - 24 x 40
=24 x (53+87-40)
=24 x 100 =2400
c,5 x 7 x 77 - 7 x 60 +49 x 25 -15 x 42.
=5 x 7 x 77 - 7 x 5 x 12 +49 x 5 x 5 - 5 x 3 x 42.
=5 x 539 - 84 x 5 +245 x 5 -5 x 126
=5 x (539 - 84 +245 -126)
=5 x 574
=2870
\(a,53\cdot39+47\cdot39-53\cdot21-47\cdot21\)
\(=39\cdot\left(53+47\right)-21\cdot\left(53+47\right)\)
\(=39\cdot100-21\cdot100\)
\(=100\cdot\left(39-21\right)=100\cdot18\)
\(=1800\)
\(b,2\cdot53\cdot12+4\cdot6\cdot87-3\cdot8\cdot40\)
\(=24\cdot53+24\cdot87-24\cdot40\)
\(=24\cdot\left(53+87-40\right)\)
\(=24\cdot100=2400\)
a/39.(53+47)-21.(53-47)
=39.100-21.6
=3900-126
=3774
b);c) tự làm
Bài làm
a) 53 . 39 + 47 . 39 - 53 . 21- 47 . 21
= 39 . ( 53 + 47 ) + 21 . ( 53 - 47 )
= 39 . ( 53 + 47 ) - 21 . ( 53 + 47 )
= ( 39 - 21 ) . ( 53 + 47 )
= 18 . 100
= 1800
b) 2 . 53 . 12 + 4 . 6 . 87 - 3 . 8 . 40
= ( 2 . 12 ) . 53 + ( 4 . 6 ) . 87 - ( 3 . 8 ) . 40
= 24 . 53 + 24 . 87 - 24 . 40
= 24 . ( 53 + 87 - 40 )
= 24 . 100
= 2400
c) 3 . 7 . 77 - 7 . 60 + 49 . 25 - 15 . 42
= 5 . ( 539 - 84 + 245 - 126 )
= 5 . 574
= 2870
# Học tốt #
