Respuesta :
Answer:
a) [tex]S(t) = 42(0.9995)^t[/tex]
b) It will take 10,692 years.
Step-by-step explanation:
Exponential function to determine the amount of substance:
An exponential function to determine the amout of substance after t years is given by:
[tex]A(t) = A(0)(1-r)^{t}[/tex]
In which A(0) is the initial amount and r is the decay rate, as a decimal.
(a) Write an equation to determine the amount of substance, S, left after t years.
Half-life of 1300 years means that [tex]A(1300) = 0.5A(0)[/tex].
We use this to find r. So
[tex]A(t) = A(0)(1-r)^{t}[/tex]
[tex]0.5A(0) = A(0)(1-r)^{1300}[/tex]
[tex](1-r)^{1300} = 0.5[/tex]
[tex]\sqrt[1300]{(1-r)^{1300}} = \sqrt[1300]{0.5}[/tex]
[tex]1 - r = 0.9995[/tex]
[tex]r = 0.0005[/tex]
Sample of 42 grams means that [tex]A(0) = 42[/tex]. So
[tex]A(t) = A(0)(1-r)^{t}[/tex]
Replacing A by S, just for notation purposes
[tex]S(t) = 42(0.9995)^t[/tex]
(b) Approximately how long will it take for 0.2 grams of substance to remain (to the nearest year)?
This is t when [tex]S(t) = 0.2[/tex]. So
[tex]S(t) = 42(0.9995)^t[/tex]
[tex]0.2 = 42(0.9995)^t[/tex]
[tex](0.9995)^t = \frac{0.2}{42}[/tex]
[tex]\log{(0.9995)^t} = \log{\frac{0.2}{42}}[/tex]
[tex]t\log{0.9995} = \log{\frac{0.2}{42}}[/tex]
[tex]t = \frac{\log{\frac{0.2}{42}}}{\log{0.9995}}[/tex]
[tex]t = 10692[/tex]
It will take 10,692 years.
