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The count of bacteria in a culture was 800 after 10 minutes and 1800 after 40 minutes. What was the initial size of the culture?

Guest Nov 24, 2014

Best Answer 

 #2
avatar+18827 
+5

The count of bacteria in a culture was 800 after 10 minutes and 1800 after 40 minutes. What was the initial size of the culture?

$$\boxed{b = b_0 * a^{t} } \\\\
b = bacteria \\
b_0=bacteria_{Initial size} \\
t= time\\
\begin{array}{lrcl}
\hline
(1) & 800 & = & b_0 * a^{10} \quad | \quad t = 10 \quad b = 800\\
(2)& 1800 & = & b_0 * a^{40} \quad | \quad t = 40 \quad b = 1800 \\
\hline
\end{array}$$

$$(2):(1) \quad \frac{1800}{800} = \frac{b_0*a^{40} } {b_0*a^{10} }\\\\
\frac{18}{8} = \frac{a^{40} } {a^{10} } = a^{40-10}=a^{30} \quad| \quad \sqrt[30]{}\\\\
\boxed{a=\sqrt[30]{ \frac{18}{8} }} \\\\
(1) \quad b_0 = \dfrac{800}{a^{10}}
=
\dfrac{800}
{
\left(\sqrt[30]{ \frac{18}{8} }\right)^{10}
} }
=
\dfrac{800}
{
\left( \frac{18}{8} }\right)^{\frac{10}{30}}
} }
=
\dfrac{800}
{
\left( \frac{18}{8} }\right)^{\frac{1}{3}}
} }
=
\dfrac{800}
{
\frac{ \sqrt[3]{18} } { \sqrt[3]{8} }
}
=
\dfrac{800}
{
\frac{ \sqrt[3]{18} } { \sqrt[3]{2^3} }
} \\\\
=
\dfrac{800}
{
\frac{ \sqrt[3]{18} } { 2 }
} =
\dfrac{1600}{ \sqrt[3]{18} } = 610.514262695$$

the initial size of the culture is 610.514262695

heureka  Nov 24, 2014
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2+0 Answers

 #1
avatar+80804 
+5

Asumiing we have a function

N = abt    where N is the bacteria count at some time t........note.....when t  = 10, N = 800....and when t = 40, N = 1800...so we have

800 = ab10     and solving for "a," we have     800/b10 = a

And we also have

1800 = ab40      and substituting, 800/b10  for a, we have

1800 = (800/b10)b40 →   1800 = 800b30

And dividing both sides by 800  we have

1800/800 = b30  →  9/4 =  b30

And taking the (positive) 30th root of each side, we have that b ≈ 1.0274

And a  = 800/(1.0274)10  ≈ 611    And this was the initial culture size  (when t = 0, b0 = 1)

 

CPhill  Nov 24, 2014
 #2
avatar+18827 
+5
Best Answer

The count of bacteria in a culture was 800 after 10 minutes and 1800 after 40 minutes. What was the initial size of the culture?

$$\boxed{b = b_0 * a^{t} } \\\\
b = bacteria \\
b_0=bacteria_{Initial size} \\
t= time\\
\begin{array}{lrcl}
\hline
(1) & 800 & = & b_0 * a^{10} \quad | \quad t = 10 \quad b = 800\\
(2)& 1800 & = & b_0 * a^{40} \quad | \quad t = 40 \quad b = 1800 \\
\hline
\end{array}$$

$$(2):(1) \quad \frac{1800}{800} = \frac{b_0*a^{40} } {b_0*a^{10} }\\\\
\frac{18}{8} = \frac{a^{40} } {a^{10} } = a^{40-10}=a^{30} \quad| \quad \sqrt[30]{}\\\\
\boxed{a=\sqrt[30]{ \frac{18}{8} }} \\\\
(1) \quad b_0 = \dfrac{800}{a^{10}}
=
\dfrac{800}
{
\left(\sqrt[30]{ \frac{18}{8} }\right)^{10}
} }
=
\dfrac{800}
{
\left( \frac{18}{8} }\right)^{\frac{10}{30}}
} }
=
\dfrac{800}
{
\left( \frac{18}{8} }\right)^{\frac{1}{3}}
} }
=
\dfrac{800}
{
\frac{ \sqrt[3]{18} } { \sqrt[3]{8} }
}
=
\dfrac{800}
{
\frac{ \sqrt[3]{18} } { \sqrt[3]{2^3} }
} \\\\
=
\dfrac{800}
{
\frac{ \sqrt[3]{18} } { 2 }
} =
\dfrac{1600}{ \sqrt[3]{18} } = 610.514262695$$

the initial size of the culture is 610.514262695

heureka  Nov 24, 2014

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