a: =>1/3*4+1/4*5+...+1/x(x+1)=10/39

=>1/3-1/4+...+1/x-1/x+1=10/39

=>1/3-1/(x+1)=10/39

=>1/(x+1)=13/39-10/39=3/39=1/13

=>x+1=13

=>x=12

b: =>15x=150

=>x=10

c: =>120-5x=45

=>5x=75

=>x=15

-12(x-5)+7(3-x)=5

=> -12x+60+21-7x=5

=> -19x+81=5

=>-19x=5-81

=>-19x=-76

=>x=4

Vậy x=4

    3/2.(x-5/3)+4/5=x+1

3/2.x-3/2.5/3+4/5=x+1

          3/2.x-5/2-x=1+4/5

          3/2.x-5/2-x=9/5

                3/2.x-x=9/5+5/2

                   1/2.x=43/10

                         x=43/10:1/2

                         x=43/5

bài này cần có đkxđ 

`@` `\text {Ans}`

`\downarrow`

\(\left(1-\dfrac{1}{2}\right)\times\left(1-\dfrac{1}{3}\right)\times\left(1-\dfrac{1}{4}\right)\times\left(1-\dfrac{1}{5}\right)...\left(1-\dfrac{1}{1000}\right)\)

`=`\(\left(\dfrac{2}{2}-\dfrac{1}{2}\right)\times\left(\dfrac{3}{3}-\dfrac{1}{3}\right)\times\left(\dfrac{4}{4}-\dfrac{1}{4}\right)...\left(\dfrac{1000}{1000}-\dfrac{1}{1000}\right)\)

`=`\(\dfrac{1}{2}\times\dfrac{2}{3}\times\dfrac{3}{4}\times...\times\dfrac{999}{1000}\)

`=`\(\dfrac{1}{1000}\)

\(=\dfrac{1}{2}.\dfrac{2}{3}.\dfrac{3}{4}.\dfrac{4}{5}.......\dfrac{99}{100}=\dfrac{1}{100}\)

phần 100 mà sao lại 1000,các bạn cố gắng giúp mình với nhé,mình cần gấp !

\(x-\left(1-x\right)=5+\left(-1+x\right)\)

\(\Leftrightarrow x-1+x=5-1+x\)

\(\Leftrightarrow2x-1=x+4\)

\(\Leftrightarrow2x-x=1+4\)

\(\Leftrightarrow x=5\)

nè

a: Ta có: ⇔x⋅14=134.4⇔x⋅14=134.4

hay x=537,6

b: Ta có: ⇔x:35=13⇔x:35=13

hay 

à quên ở đây ko viết phân số đc đừng viết nha

a/\(0,25.x:3=\dfrac{5}{6}=>0,25.x=\dfrac{5}{6}.3=2,5\)

\(=>x=2,5:0,25=10\)

b/\(\dfrac{-2}{5}.x=\left(\dfrac{-2}{5}\right)^3=>\dfrac{-2}{5}.x=\dfrac{-8}{125}\)

=>x=\(\dfrac{-8}{125}:\left(\dfrac{-2}{5}\right)\)=\(\dfrac{4}{25}\)