Question

So sánh:

C = \(\frac{1}{4^1}+\frac{1}{4^2}+\frac{1}{4^3}+...+\frac{1}{4^{999}}+\frac{1}{4^{1000}}\) và \(\frac{1}{3}\)

Answer 1

Ta có :

\(C=\frac{1}{4}+\frac{1}{4^2}+.....+\frac{1}{4^{1000}}\)

\(\Rightarrow4C=1+\frac{1}{4}+.....+\frac{1}{4^{1999}}\)

\(\Rightarrow4C-C=\left(1+\frac{1}{4}+.....+\frac{1}{4^{1999}}\right)-\left(\frac{1}{4}+\frac{1}{4^2}+.....+\frac{1}{4^{1000}}\right)\)

\(\Rightarrow3C=1-\frac{1}{4^{1000}}\)

\(\Rightarrow C=\frac{1}{3}-\frac{1}{3.4^{1000}}< \frac{1}{3}\)

=> C < 1 / 3

Answer 2

Ta có:

\(C=\frac{1}{4}+\frac{1}{4^2}+...+\frac{1}{4^{1000}}\)

\(\Rightarrow4C=1+\frac{1}{4}+...+\frac{1}{4^{999}}\)

\(\Rightarrow4C-C=\left(1+\frac{1}{4}+\frac{1}{4^2}+...+\frac{1}{4^{999}}\right)-\left(\frac{1}{4}+\frac{1}{4^2}+...+\frac{1}{4^{999}}+\frac{1}{4^{1000}}\right)\)

\(\Rightarrow3C=1-\frac{1}{4^{1000}}\)

\(\Rightarrow C=\left(1-\frac{1}{4^{1000}}\right).\frac{1}{3}\)

\(\Rightarrow C=\frac{1}{3}-\frac{1}{4^{1000}.3}\)

Mà \(\frac{1}{3}>\frac{1}{3}-\frac{1}{4^{1000}.3}\)

\(\Rightarrow C< \frac{1}{3}\)

Vậy \(C< \frac{1}{3}\)