a, -x/4 = -9/x

=> -x.x = 4.(-9)

=> -x² = -36

=> x² = 36

=> x = + 4

vậy_

b, x/4 = 18/x + 1

=> x(x + 1) = 4.18

=> x(x + 1) = 72

=> x(x + 1) = 8.9

=> x = 8

vậy_

a)\(\frac{-x}{4}=\frac{-9}{x}\)

\(\Rightarrow-x\times x=-9\times4\)

\(\Rightarrow-x^2=-36\)

\(\Rightarrow x=6\)

bài không khó đâu bn à

=> x + x - 1 + x -2 =2

=> 3x - 3 = 2

=> 3x = 2 +3

=> 3x = 5

=> x = 5/3

Làm bạn

mk chọn 1+1=2 nha

3X=-7-13+5=-15

Vậy X=-5
 

\(3x-5=-7-13\)

\(\Rightarrow3x-5=-20\)

\(\Rightarrow3x=(-20)+5\)

\(\Rightarrow3x=-15\)

\(\Rightarrow x=-5\)

A) -a+(-b)=27+16=43

B) a-b=-43

C) -a.(-b)=432

D) \(\frac{a}{3}\)+\(\frac{-b}{4}\)=-9+4=-5

a) \(\frac{x}{4}=\frac{21}{28}\Rightarrow\frac{x}{4}=\frac{3}{4}\Rightarrow x=3\)

Vậy x = 3

b) \(\frac{2x}{5}=\frac{-24}{10}\Rightarrow\frac{2x}{5}=\frac{-12}{5}\Rightarrow2x=-12\Rightarrow x=-6\)

Vậy x = -6

\(a)\frac{x}{4}=\frac{21}{28}\)

\(\Rightarrow x\cdot28=4\cdot21\)

\(\Rightarrow x\cdot28=84\)

\(\Rightarrow x=3\)

\(b)\frac{2x}{5}=\frac{-24}{10}\)

Rút gọn : \(\frac{2x}{5}=\frac{-12}{5}\)

\(\Rightarrow2x=-12\)

\(\Rightarrow x=(-12)\div2=-6\)

1!+2.2!+3.3!+............+n.n!

<=> (n+1)!-1

Đề bài là gì hả bạn

làm gì ? đề bài ?

\(\frac{1}{7}+\frac{1}{91}+\frac{1}{247}+\frac{1}{475}+\frac{1}{775}+\frac{1}{1147}\)

\(=\frac{1}{1\cdot7}+\frac{1}{7\cdot13}+\frac{1}{13\cdot19}+...+\frac{1}{31\cdot37}\)

\(=\frac{1}{6}\left(\frac{6}{1\cdot7}+\frac{6}{7\cdot13}+\frac{6}{13\cdot19}+...+\frac{6}{31\cdot37}\right)\)

\(=\frac{1}{6}\left(1-\frac{1}{7}+\frac{1}{7}-\frac{1}{13}+\frac{1}{13}-\frac{1}{19}+...-\frac{1}{31}-\frac{1}{37}\right)\)

\(=\frac{1}{6}\left(1-\frac{1}{37}\right)\)

\(=\frac{1}{6}\cdot\frac{36}{37}=\frac{6}{37}\)

Tổng cần tính bằng:\(\frac{1}{1.7}\)+\(\frac{1}{7.13}\)+\(\frac{1}{13.19}\)+\(\frac{1}{19.25}\)+\(\frac{1}{25.31}\)+\(\frac{1}{31.37}\)=(\(\frac{1}{1}\)-\(\frac{1}{7}\)+\(\frac{1}{7}\)-\(\frac{1}{13}\)+...+\(\frac{1}{31}\)\(\frac{1}{37}\)):3 =(\(1\)-\(\frac{1}{37}\)):3=\(\frac{12}{37}\)