Exponential functions are different from other functions studied in this course because the variable appears as the exponent instead of the base. In general an exponential function takes the form: y = a · b^{x}, where x is a real number, a ≠ 0, b > 0, and b ≠ 1. The value of a is the initial amount, or y-intercept, and b is the common ratio. It is called a common ratio because every time x is increased by a factor of one, the expression gets multiplied by the value of b.