Infinite Decimals

Algebra Level 3

0.111 ÷ 0.222 × 0.333 ÷ 0.444 × 0.555 ÷ 0.666 × 0.777 ÷ 0.888 × 0.999 0.111\dots \quad \div \quad 0.222\dots \quad \times \quad 0.333\dots \quad \div \quad 0.444\dots \quad \times \quad 0.555\dots \quad \div \quad 0.666\dots \quad \times \quad 0.777\dots \quad \div \quad 0.888\dots \quad \times \quad 0.999

Compute the number above and round your answer to 3 decimal places.


The answer is 0.273.

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2 solutions

Rishabh Jain
Feb 10, 2016

( 0.111 ÷ 0.222 ) × ( 0.333 ÷ 0.444 ) × ( 0.555 ÷ 0.666 ) × ( 0.777 ÷ 0.888 ) × 0.999 \color{#007fff}{(0.111\dots \quad \div \quad 0.222\dots)} \quad \times \quad\color{#007fff}{(0.333\dots \quad \div \quad 0.444\dots)} \quad \times \quad\color{#007fff}{(0.555\dots \quad \div \quad 0.666\dots)} \quad \times \quad\color{#007fff}{(0.777\dots \quad \div \quad 0.888\dots)} \quad \times \quad0.999 = 1 2 × 3 4 × 5 6 × 7 8 × 0.999 =\color{#007fff}{\dfrac{1}{2}}\times \color{#007fff}{\dfrac{3}{4}}\times \color{#007fff}{\dfrac{5}{6}}\times \color{#007fff}{\dfrac{7}{8}}\times 0.999 = 35 × 0.999 128 = 0.273 =\dfrac{35\times 0.999}{128}=\color{#0C6AC7}{\boxed{\color{#D61F06}{0.273}}} (to 3 decimal places) ~~~~~~\small\color{#456461}{{\text{(to 3 decimal places)}}}

x = 0.111... \Rightarrow x=0.111... .... ( 1 ) (1)
10 x = 1.111... \Rightarrow 10x=1.111... .... ( 2 ) (2)

Subtracting ( 1 ) (1) from ( 2 ) (2) .

9 x = 1 \Rightarrow 9x=1
x = 1 9 \Rightarrow x=\color{#BA33D6}{\frac{1}{9}}

Similarly,

0.111... = 1 9 \Rightarrow 0.111...= \color{#BA33D6}{\frac{1}{9}}

0.222... = 2 9 \Rightarrow 0.222...=\color{#302B94}{\frac{2}{9}}

0.333... = 3 9 \Rightarrow 0.333...=\color{#3D99F6}{\frac{3}{9}}

0.444... = 4 9 \Rightarrow 0.444...=\color{#20A900}{\frac{4}{9}}

0.555... = 5 9 \Rightarrow 0.555...=\color{#624F41}{\frac{5}{9}}

0.666... = 6 9 \Rightarrow 0.666...=\color{#EC7300}{\frac{6}{9}}

0.777... = 7 9 \Rightarrow 0.777...=\color{#D61F06}{\frac{7}{9}}

0.888... = 8 9 \Rightarrow 0.888...=\color{#CEBB00}{\frac{8}{9}}

0.999... = 9 9 \Rightarrow 0.999...=\color{magenta}{\frac{9}{9}}

Now,

( 1 9 × 9 2 ) × ( 3 9 × 9 4 ) × ( 5 9 × 9 6 ) × ( 7 9 × 9 8 ) × 9 9 \Rightarrow (\color{#BA33D6}{\frac{1}{9}}×\color{#302B94}{\frac{9}{2}})×(\color{#3D99F6}{\frac{3}{9}}×\color{#20A900}{\frac{9}{4}})×(\color{#624F41}{\frac{5}{9}}×\color{#EC7300}{\frac{9}{6}})×(\color{#D61F06}{\frac{7}{9}}×\color{#CEBB00}{\frac{9}{8}})×\color{magenta}{\frac{9}{9}}

3 × 5 × 7 2 × 4 × 6 × 8 = 35 128 0.273 \Rightarrow \color{grey}{\frac{3×5×7}{2×4×6×8}}=\color{grey}{\frac{35}{128}}\approx\color{#D61F06}{\boxed{\color{#3D99F6}{0.273}}}

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