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How do I solve an endless amount of numbers with a common ratio
..........

etc. 10+1+0.1+0.01+0.001....


Sagot :

Answer:

11 .21 add the number together

Notice that the given sum is equivalent to

[tex]10 + 1 + \dfrac1{10} + \dfrac1{10^2} + \dfrac1{10^3} + \cdots \\\\ ~~~~~~~~~~~~ = 10 \left(1 + \dfrac1{10} + \dfrac1{10^2} + \dfrac1{10^3} + \dfrac1{10^4} + \cdots\right)[/tex]

or 10 times the infinite geometric series with ratio 1/10.

To compute the infinite sum, consider the [tex]n[/tex]-th partial sum

[tex]S_n = 1 + \dfrac1{10} + \dfrac1{10^2} + \cdots + \dfrac1{10^{n-1}}[/tex]

Multiply both sides by the ratio.

[tex]\dfrac1{10} S_n = \dfrac1{10} + \dfrac1{10^2} + \dfrac1{10^3} + \cdots + \dfrac1{10^n}[/tex]

Subtract this from [tex]S_n[/tex] to eliminate all but the outermost terms,

[tex]S_n - \dfrac1{10} S_n = 1 - \dfrac1{10^n} \implies S_n = \dfrac{10}9 \left(1 - \dfrac1{10^n}\right)[/tex]

As [tex]n\to\infty[/tex], the exponential term will decay to 0, leaving us with

[tex]1 + \dfrac1{10} + \dfrac1{10^2} + \cdots = \dfrac{10}9[/tex]

Then the value of the sum we want is

[tex]10\times\dfrac{10}9 = \boxed{\dfrac{100}9}[/tex]