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Find the probability that a given light bulb lasts between 750 and 900 hours.

Be sure to use the [tex]$68 \%-95 \%-99.7 \%$[/tex] rule and do not round.


Sagot :

Sure! Let's solve this problem step-by-step.

### Given:
- Mean life of the light bulb ([tex]\(\mu\)[/tex]) = 800 hours
- Standard deviation ([tex]\(\sigma\)[/tex]) = 40 hours
- Lower bound = 750 hours
- Upper bound = 900 hours

### Step-by-Step Solution:

1. Calculate the z-scores for the lower and upper bounds using the formula for the z-score:
[tex]\[ z = \frac{X - \mu}{\sigma} \][/tex]

- For the lower bound (750 hours):
[tex]\[ z_{\text{lower}} = \frac{750 - 800}{40} = \frac{-50}{40} = -1.25 \][/tex]

- For the upper bound (900 hours):
[tex]\[ z_{\text{upper}} = \frac{900 - 800}{40} = \frac{100}{40} = 2.5 \][/tex]

2. Find the cumulative distribution function (CDF) values for these z-scores:

- [tex]\( P(Z \leq -1.25) \approx 0.10564977366685535 \)[/tex]
- [tex]\( P(Z \leq 2.5) \approx 0.9937903346742238 \)[/tex]

3. Calculate the probability that the light bulb lasts between 750 and 900 hours:
[tex]\[ P(750 \leq X \leq 900) = P(Z \leq 2.5) - P(Z \leq -1.25) \approx 0.9937903346742238 - 0.10564977366685535 = 0.8881405610073685 \][/tex]

### Conclusion:
The probability that a given light bulb lasts between 750 and 900 hours is approximately [tex]\(0.8881\)[/tex], or 88.81%.

This solution includes using the z-scores and the cumulative distribution function to find the probability accurately without rounding. This ensures the precision of our result.