Use Excel to find the z-scores that bound the middle $90 \%$ of the area under the standard normal curve. Enter the answers in ascending order. Round the answers to two decimal places.
The $z$-scores for the given area are and
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Step 1 ：The question asks for the z-scores that bound the middle 90% of the area under the standard normal curve. The standard normal curve is a normal distribution with a mean of 0 and a standard deviation of 1. The z-score is a measure of how many standard deviations an element is from the mean.

Step 2 ：To find the z-scores that bound the middle 90% of the area under the standard normal curve, we need to find the z-scores that leave 5% of the distribution on either side (since 100% - 90% = 10%, and we split this evenly on either side of the distribution).

Step 3 ：The z-scores for the given area are -1.64 and 1.64.

Step 4 ：Final Answer: The z-scores that bound the middle 90% of the area under the standard normal curve are -1.64 and 1.64. So, the final answer is \(\boxed{-1.64, 1.64}\).