\(a.\dfrac{6}{7}-\dfrac{4}{21}=\dfrac{18-4}{21}=\dfrac{14}{21}=\dfrac{2}{3}\\ b.6\times\dfrac{12}{18}=\dfrac{6\times12}{18}=\dfrac{4}{1}=4\\ c.\dfrac{2}{5}\times\dfrac{3}{7}+\dfrac{2}{5}\times\dfrac{4}{7}-\left(\dfrac{3}{7}+\dfrac{4}{7}\right)\times\dfrac{2}{5}=\dfrac{7}{7}\times\dfrac{2}{5}=1\times\dfrac{2}{5}=\dfrac{2}{5}\)

18/21 - 4/21 = 14/21 = 2/3

(3/7+4/7)x 2/5= 7/7x 2/5 = 1x2/5 = 2/5

Đề yêu cầu gì vậy em, phải có yêu cầu cụ thể mới cứu em được chứ nhỉ?

pt <=> x^4-6x^2-12x-6-8 = 0

<=> x^4-6x^2-12x-14 = 0

\(\dfrac{1}{56}+\dfrac{1}{72}+\dfrac{1}{90}+\dfrac{1}{110}\)

\(=\dfrac{1}{7\cdot8}+\dfrac{1}{8\cdot9}+\dfrac{1}{9\cdot10}+\dfrac{1}{10\cdot11}\)

\(=\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{11}\)

\(=\dfrac{1}{7}-\dfrac{1}{11}\)

\(=\dfrac{11}{77}-\dfrac{7}{77}\)

\(=\dfrac{4}{77}\)

\(=\dfrac{1}{7.8}+\dfrac{1}{8.9}+\dfrac{1}{9.10}+\dfrac{1}{10.11}=\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{11}\)

\(=\dfrac{1}{7}-\dfrac{1}{11}=\dfrac{11}{77}-\dfrac{7}{77}=\dfrac{4}{77}\)

a,\(4:\dfrac{9}{5}:\dfrac{10}{3}\)
\(=\dfrac{4\times5\times3}{9\times10}\)
\(=\dfrac{2\times2\times5\times3}{3\times3\times5\times2}=\dfrac{2}{3}\)
b,\(\dfrac{1}{2}\times\dfrac{3}{4}:\dfrac{6}{5}\)
\(=\dfrac{3\times5}{2\times4\times6}\)
\(=\dfrac{3\times5}{2\times4\times3\times2}=\dfrac{5}{16}\)

b) Đáp án:

0.3125

Giải thích các bước giải:

1/2 x 3/4 : 6/5 

=3/8 : 6/5 

= 5/16

=0.3125

Mình gửi thêm nha!

a) $4\; :\frac{9}{\mathrm{/5}}:\frac{\mathrm{10/}}{3}$

$=4\times \frac{\mathrm{5/}}{9}:\frac{10}{\mathrm{/3}}$

$=\frac{4}{\mathrm{/1}}\times \frac{5}{\mathrm{/9}}:\frac{10}{\mathrm{/3}}$

$=\frac{20}{\mathrm{/9}}\times \frac{3}{\mathrm{/10}}$

$=\frac{60}{\mathrm{/90}}$

$=\frac{60:30}{}$$\frac{90:30}{}$

$=\frac{2}{\mathrm{/3}}$

2 + x + 4 + x + ....... + 52 + x = 780

(x + x + .....  x) + (2 + 4 + 6 + ...... + 52) = 780

26 * x + (52 + 2) x 26 : 2 = 780

26 * x + 702 = 780

26 * x = 780 - 720

26 * x = 78

x = 78 : 26

x=  3 

x⁴+4-36x²+36-x⁴+2x²-2x²-4

=(x⁴-x⁴)+(4-4)+(2x²-2x²-36x²)+36

=0+0-36x²+36

=còn lai dặt thừa số chung nha mình phải đi học rùi bye bye hêhhe

(x + 1)⁴ - 6(x + 1)² - (x² - 2)(x² + 2)

= (x² + 2x + 1)(x² + 2x + 1) - 6(x² + 2x + 1) - (x² - 2)(x² + 2)

= x⁴ + 2x³ + x² + 2x³ + 4x² + 2x + x² + 2x + 1 - 6x² - 12x - 6 - x⁴ + 4

= 4x³ - 8x - 1

Ta có: \(\left(2+4+6+...+2004\right)\cdot x=501396\)

\(\Leftrightarrow\frac{\left(2+2004\right)\cdot\left[\left(2004-2\right)\div2+1\right]}{2}\cdot x=501396\)

\(\Leftrightarrow\frac{2006\cdot1002}{2}\cdot x=501396\)

\(\Leftrightarrow1005006\cdot x=501396\)

\(\Rightarrow x=\frac{250698}{502503}\)

Lời giải:
** Bổ sung điều kiện $x,y$ là số nguyên.

Với $x,y$ nguyên thì $3x+2, y-4$ cũng nguyên.

Mà $(3x+2)(y-4)=6$ nên ta có bảng sau: