In science, an unknown value is represented by a letter in the Roman or Greek alphabet. They are used most often in physics, where equations are used to describe the relationship between physical properties. For example, in the equation , the letter represents an unknown (in this case, mass), and the letter also represents an unknown (in this case the amount of energy). If we know the value of all but one of the unknowns, then we would be able to easily find out the value of the last unknown—by solving the equation in terms of that last unknown.
When letters in science are used this way, not all of them represent unknowns. Indeed, some of them might be physical constants, which are values that are known to be the same and do not change. In the example above, represents a known quantity, the speed of light, which is about 186 thousand miles per second. And because is so large, the formula tells a person who understands algebra that in an atomic reaction, a small amount of mass can produce a very large amount of energy.
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