.3-2/2.3 + 4-3/3.4 + 5-4/4.5 + 6-5/5.6 +...+ 20-19/19.20=18/x
1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 +...+ 1/19 - 1/20=18/x
1/2 - 1/20=18/x
10/20 - 1/20=18/x
9/20=18/x
18/40=18/x
=>x=40
Vậy x=40
1/2.3 + 1/3.4 + 1/4.5 + ... + 1/18.19 + 1/19.20
\(A=\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+....+\dfrac{1}{19.20}\)
tìm x, biết
1/3.4+1/4.5+1/5.6+1/6.7+...+1/x.(x+1)=3/10
1/2.3+1/3.4+1/4.5+1/5.6+1/6.7
\(\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}\)
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6
tìm x biết : \(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+.....+\frac{1}{x\left(x+1\right)}=\frac{17}{18}\)
tìm x biết :\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{19.20}-\frac{x}{40}=\frac{3}{-10}\)
Tính 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 +1/5.6
