Bài 2 trang 55 Toán 6 Cánh diều: Cho 23, 456 = 10 488. Tính nhẩm: *20/l/a) 2, 3. 456;b) 2, 3. 45, 6;c) – 2, 3 . – 4, 56 ;d) – 2, 3

Đề bài/câu hỏi:

Cho \(23,456 = 10{\rm{ }}488.\) Tính nhẩm:

\(\begin{array}{*{20}{l}}{a){\rm{ }}2,3.456\;}\\{b){\rm{ }}2,3.45,6\;}\\{c){\rm{ }}\left( { – 2,3} \right).\left( { – 4,56} \right)\;}\\{d){\rm{ }}\left( { – 2,3} \right).45{\rm{ }}600}\end{array}\)

Hướng dẫn:

1) Tính chất giao hoán: a.b = b.a

2) Tính chất kết hợp: (a.b).c = a.(b.c)

Lời giải:

\(\begin{array}{l}a){\rm{ }}2,3{\rm{ }}.{\rm{ }}456 = 23.0,1.456{\rm{ }}\\ = 23.456.0,1 =10488.0,1= 10{\rm{ }}48,8.\end{array}\)

\(\begin{array}{l}b){\rm{ }}2,3.45,6 = \left( {23.0,1} \right).\left( {456.0,1} \right)\\ = \left( {23.456} \right).\left( {0,1.0,1} \right){\rm{ }}\\ = 10{\rm{ }}488.0,01\\ = 104,88\end{array}\)

\(\begin{array}{l}c){\rm{ }}\left( { – 2,3} \right).\left( { – 4,56} \right){\rm{ }}\\ = \left( { – 23.0,1} \right).\left( { – 456.0,01} \right){\rm{ }}\\ = 10{\rm{ }}488.0,001\\ = 10,488\end{array}\)

\(\begin{array}{l}d){\rm{ }}\left( { – 2,3} \right).45{\rm{ }}600{\rm{ }}\\ = \left( { – 23.0,1} \right).\left( {456.100} \right){\rm{ }}\\ = \left( { – 23.456} \right).\left( {0,1.100} \right){\rm{ }}\\ = – 10{\rm{ }}488.10{\rm{ }}\\ = – 104{\rm{ }}880\end{array}\)