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.
The three germ layers are the ectoderm (outer layer), mesoderm (middle layer), and endoderm (inside layer). Cnidarians only have an ectoderm and an endoderm. It is thought that diploblastic animals evolved very early in the history of multicellular life, and may have actually been the first true multicellular organisms.