Question

[Nguyễn Thư Thư]

bài 1 : thực hiện phép tính ( hợp lí )

a, (375 - 180) : 15

b, 678. (276+ 49)- 678 .(149 +76 )

bài 2:tính giá trị của biểu thức

12:\(\left\{390:\left[500-\left(125+35.7\right)\right]\right\}\)

bài 3 : tìm x thuộc N

a,2.\(3^x=54\)

b,\(3^2.3^x=81\)

c,\(\left(x-1\right)^3=8\)

bài 4; tính nhanh

A=100+98+96+....+2-97-95-....-1

Answer 1

[Phạm Ngân Hà]

Bài 1:

a) \(\left(375-180\right):15\)

\(=195:15\)

\(=13\)

b) \(678\left(276+49\right)-678\left(149+76\right)\)

\(=678.325-678.225\)

\(=678\left(325-225\right)\)

\(=678.100\)

\(=67800\)

Bài 2:

\(12:\left\{390:\left[500-\left(125+35.7\right)\right]\right\}\)

\(=12:\left\{390:\left[500-\left(125+245\right)\right]\right\}\)

\(=12:\left\{390:\left[500-370\right]\right\}\)

\(=12:\left\{390:130\right\}\)

\(=12:3\)

\(=4\)

Bài 3:

a) \(2.3^x=54\)

\(\Leftrightarrow3^x=54:2\)

\(\Leftrightarrow3^x=27\)

\(\Leftrightarrow3^x=3^3\)

\(\Leftrightarrow x=3\)

b) \(3^2.3^x=81\)

\(\Leftrightarrow9.3^x=81\)

\(\Leftrightarrow3^x=81:9\)

\(\Leftrightarrow3^x=9\)

\(\Leftrightarrow3^x=3^2=\left(-3\right)^2\)

\(\Leftrightarrow x=2\)

c) \(\left(x-1\right)^3=8\)

\(\Leftrightarrow\left(x-1\right)^3=2^3\)

\(\Leftrightarrow x-1=2\)

\(\Leftrightarrow x=2+1\)

\(\Leftrightarrow x=3\)

Bài 4:

\(A=100+98+96+...+2-97-95-...-1\)

\(\Leftrightarrow A=\left(100+98+96+...+2\right)-\left(97+95+...+1\right)\)

\(\Leftrightarrow A=\left[\left(100+2\right).50:2\right]-\left[\left(97+1\right).49:2\right]\)

\(\Leftrightarrow A=2550-2401\)

\(\Leftrightarrow A=149\)