\(A=1.2+2.3+........+19.20\)

\(3A=3.1.2+3.2.3+.........+3.19.20\)

\(3A=\left(3-0\right).1.2+\left(4-1\right).2.3+..........+\left(21-18\right).19.20\)

\(3A=1.2.3-0.1.2+2.3.4-1.2.3+........+19.20.21-18.19.20\)

\(3A=19.20.21\)

\(3A=7980\)

Chú thích : dấu . là dấu nhân nha

= ( 25 * 4 ) * 14 

=  100 * 14 

= 1400

=25x4x14

=100x14

=1400

rút gọn là được mà ! 

\(\frac{1}{2}\)

x×2+x×3+x×4+x=2013

x×(1+2+3+4)=2013

x×10=2013

⇔x=2013/10 

\(x\times2+x\times3+x\times4+x=2013\)

\(x\times\left(1+2+3+4\right)=2013\)

\(x\times10=2013\)

\(\Leftrightarrow x=\frac{2013}{10}\)

P/s tham khảo nha

Bài 1

245+368+155+332/25.4

=(245+155)+(368+332)/100

=400+700/100

=1100/100

=11/1=11

Bài 2

X:6+234578=20208+197356+39792+202644

X:6+234578=(20208+39792)+(197356+202644)

X:6+234578=60000+400000=460000      (bước này mk rút gọn cho dễ, nhưng trong tập thì bạn có thể làm ra từng bước nha)

X:6=460000-234578

X:6=225422

X   =225422.6

X   =1 352 532.

\(a,\frac{3\times4\times7}{12\times8\times9}\)

\(=\frac{3\times4\times7}{3\times4\times8\times9}\)

\(=\frac{7}{72}\)

\(b,\frac{4\times5\times6}{12\times10\times8}\)

\(=\frac{4\times5\times3\times2}{4\times3\times5\times2\times8}\)

\(=\frac{1}{8}\)

\(c,\frac{5\times6\times7\times9}{12\times7\times27}\)

\(=\frac{5\times6\times7\times9}{6\times2\times7\times9\times3}\)

\(=\frac{5}{6}\)

\(d,\frac{5\times6\times7}{12\times14\times15}\)

\(=\frac{5\times6\times7}{6\times2\times7\times2\times5\times3}\)

\(=\frac{1}{12}\)

a) \(\frac{3\times4\times7}{12\times8\times9}=\frac{3\times4\times7}{3\times4\times8\times9}=\frac{7}{8\times9}=\frac{7}{72}\)

b) \(\frac{4\times5\times6}{12\times10\times8}=\frac{4\times5\times6}{6\times2\times2\times5\times4\times2}=\frac{1}{2\times2\times2}=\frac{1}{8}\)

c) \(\frac{5\times6\times7\times9}{12\times7\times27}=\frac{5\times6\times9}{12\times27}=\frac{5\times6\times9}{2\times6\times3\times9}=\frac{5}{2\times3}=\frac{5}{6}\)

d) \(\frac{5\times6\times7}{12\times14\times15}=\frac{5\times6\times7}{2\times6\times2\times7\times3\times5}=\frac{1}{2\times2\times3}=\frac{1}{12}\)

2x3x4x8x50x25x125

=(2x50)x(4x25)x(8x125)x3

=100x100x1000x3

=10000000x3=30000000

30000000 nha minh ban k cho mik mik se tra lai

=\(\frac{3\times3\times3\times5\times3\times7\times2\times6\times4\times5}{5\times2\times3\times5\times3\times3\times3\times6\times4}=7\)

\(\frac{9x15x21x12x20}{5x6x45x18x4}=\frac{3x3x15x3x7x3x4x5x4}{5x6x15x3x3x3x2x4}=\frac{15x7x3x4}{6x2}=\frac{1260}{12}=105\)

nhân biểu thức vs 2. đcj bao nhiêu chia cho 2