We can't use both. The answer is to use the one with the smaller absolute value: With this definition of. A slightly different formal definition of the limit is used when x goes to +/- infinity, since we really cannot subtract x from infinity or vice versa-remember, infinity is a concept, not a number. Formally, the expression. Means that.
During the long interval of the Precambrian Era (which includes approximately 90% of geologic time and includes three Eons: the Hadean, the Archean and the Proterozoic) the only inhabitants of the Earth were simple microscopic organisms, many of them comparable in size and complexity to modern-day bacteria.
During the Tertiary Period the dominant organisms were mammals.  These mammals included the many different types of dinosaurs that  were roaming the earth 65 million years a â¦ go. The Devonian Period, from the Paleozoic Era, was also known as the Age of the Fishes. 2  At thâ¦. 3  Domain Archaea: a New Discovery Organisms from the domain archaea are microscopic organisms that were just discovered in the 1970s.