Question

Tìm \(x\in Z\), biết:

                   \(\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{x\left(x+1\right)}=\frac{3}{10}\)

Answer 1

Quá dễ:

=> 1/3 - 1/4 + 1/4 - 1/5 + ....+ 1/x - 1/x+1 = 3/10

=> 1/3 - 1/x+1 = 3/10

=> 1/x+1 = 1/3 - 3/10

Còn lại tự làm nhá!

Answer 2

<=> 1/3 - 1/(x+1) = 3/10 

<=> 1/(x+1) = 1/30

=> x+1 = 30 

<=> x= 29 

Answer 3

= > 1/3 - 1/4 +1/4 -1/5 + ..... + 1/x - 1/x+1 = 3/10

= > 1/3 - 1/x+1 = 3/10

= > 1/x+1         = 1/3 - 3/10

= > 1/x+1         =1/30 

= > x+1 = 30

= > x     = 29

Vậy x = 29

Answer 4

1/3.4 + 1/4.5 + 1/5.6 + ... + 1/x (x+1) = 3/10

1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + ... + 1/x - 1/x+1 = 3/10

1/3 - 1/x+1 = 3/10

1/x+1 = 1/3 - 3/10 

1/x+1 = 1/30

=> x+1 = 30

x = 30-1

x = 29