In an exponential function of the form f(x) = a*b^x, the initial value is usually taken to be the value of f(0), or "a". The common ratio refers to the rate of change in an exponential function. In the form given above, the common ratio is "b". For example, in the function f(x) = 2*3^x, the initial value is 2 and the common ratio is 3.
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