Its basically a fancy word for fainting. If you're exercising in high heat that you're not used to, the heat can cause a drastic drop in blood pressure. Thus, you will pass out. Just take it easy, and you'll lower the risk of this happening to you.
The mean is the sum of all elements in the sequence divided by the total number of elements. It is a statistic which represents the expected value of any term in the sequence (the average value).
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