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At the beginning of an experiment, a scientist has 236 grams of radioactive goo. After 90 minutes, her sample has decayed to 7.375 grams.

1. What is the half-life of the goo in minutes? [tex]$\square$[/tex]

2. Find a formula for [tex]$G(t)$[/tex], the amount of goo remaining at time [tex]$t$[/tex].
[tex]\[ G(t) = \square \][/tex]

3. How many grams of goo will remain after 8 minutes? [tex]$\square$[/tex]
You may enter the exact value or round to 2 decimal places.


Sagot :

Sure, let's walk through the solution step-by-step.

### Step 1: Determine the Half-Life of the Goo

The scientist begins with 236 grams of radioactive goo. After 9 minutes, the amount of goo decays to 7.375 grams.

To find the half-life, [tex]\( T \)[/tex], we use the radioactive decay formula:

[tex]\[ G(t) = G_0 \left(\frac{1}{2}\right)^{\frac{t}{T}} \][/tex]

where:
- [tex]\( G(t) \)[/tex] is the remaining amount of goo at time [tex]\( t \)[/tex],
- [tex]\( G_0 \)[/tex] is the initial amount of goo,
- [tex]\( T \)[/tex] is the half-life.

Given that [tex]\( G_0 = 236 \)[/tex] grams, [tex]\( G(9) = 7.375 \)[/tex] grams, and [tex]\( t = 9 \)[/tex] minutes, we substitute these values into the equation:

[tex]\[ 7.375 = 236 \left(\frac{1}{2}\right)^{\frac{9}{T}} \][/tex]

Taking the natural logarithm on both sides to solve for [tex]\( T \)[/tex]:

[tex]\[ \ln(7.375) = \ln(236 \left(\frac{1}{2}\right)^{\frac{9}{T}}) \][/tex]

This simplifies to:

[tex]\[ \ln(7.375) = \ln(236) + \frac{9}{T} \ln\left(\frac{1}{2}\right) \][/tex]

From this, we can isolate [tex]\( T \)[/tex]:

[tex]\[ \frac{9}{T} \ln\left(\frac{1}{2}\right) = \ln(7.375) - \ln(236) \][/tex]

Solving for [tex]\( T \)[/tex]:

[tex]\[ T = \frac{9 \ln\left(\frac{1}{2}\right)}{\ln(7.375) - \ln(236)} \][/tex]

Given our constants:
- [tex]\(\ln(7.375) \approx 1.9981\)[/tex],
- [tex]\(\ln(236) \approx 5.4638\)[/tex],
- [tex]\(\ln\left(\frac{1}{2}\right) \approx -0.6931\)[/tex],

So:

[tex]\[ T = \frac{9 \times (-0.6931)}{1.9981 - 5.4638} \approx \frac{9 \times (-0.6931)}{-3.4657} \][/tex]

[tex]\[ T \approx 1.7999 \][/tex]

Therefore, the half-life of the goo is approximately [tex]\( 1.80 \)[/tex] minutes.

### Step 2: Derive the Formula for [tex]\( G(t) \)[/tex]

Using the computed half-life [tex]\( T \approx 1.80 \)[/tex], we can write the decay formula for the remaining amount of goo at time [tex]\( t \)[/tex]:

[tex]\[ G(t) = 236 \left(\frac{1}{2}\right)^{\frac{t}{1.80}} \][/tex]

### Step 3: Compute the Amount of Goo Remaining After 8 Minutes

To find the remaining amount of goo after 8 minutes, substitute [tex]\( t = 8 \)[/tex] into the decay formula:

[tex]\[ G(8) = 236 \left(\frac{1}{2}\right)^{\frac{8}{1.80}} \][/tex]

Simplify the exponent:

[tex]\[ G(8) = 236 \left(\frac{1}{2}\right)^{4.4444} \][/tex]

Using our calculations:

[tex]\[ G(8) \approx 10.8393 \][/tex]

So, the amount of goo remaining after 8 minutes is approximately [tex]\( 10.84 \)[/tex] grams.

In summary:

1. The half-life of the goo is approximately [tex]\( \boxed{1.80} \)[/tex] minutes.
2. The formula for [tex]\( G(t) \)[/tex] is [tex]\( \boxed{G(t) = 236 \left(\frac{1}{2}\right)^{\frac{t}{1.80}}} \)[/tex].
3. The amount of goo remaining after 8 minutes is approximately [tex]\( \boxed{10.84} \)[/tex] grams.
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