6.1 Introduction: The functions y = xn are power functions, so polynomials are made from power functions. More general exponents arise naturally in applications. For example, volume increases as the (3/2)th power of the surface area. "Root" functions such as the square root of x and cube root of x are power functions.
Power functions are functions of the form f(x) = x2, f(x) = 4 x-3, f(x) = -9 x1/2, and so on. The general form is f(x)= kxp where p is any real number and k is non-zero.
6.2 Domains of power function: If p is a non-zero integer, then the domain of the power function f(x) = kxp consists of all real numbers. For rational exponents p, xp is always defined for positive x, but we cannot extract an even root of a negative number. Thus x(1/4) is not defined for any negative real numbers. Neither is x(3/4) (the fourth root of x cubed).
Any rational number p can be written in the form p = r/s where all common factors of r and s have been cancelled. When this has been done, kxp has domain
If p is a real number which is not rational (called an irrational number), then the domain of xp consists of all non-negative real numbers
6.3 Logarithm as power function: Logarithms give us away around this as the Power Function is in fact an Antilogarithm. The math behind writing a power function, using the above rules, is as follows:
ln(xy) = y x ln(x)
xy = ey x ln(x)
Some basic log rules:
Logarithms only exist for Positive values, i.e. values that are strictly greater than zero. 0 raised to any power is defined as 0, and negative numbers raised to powers are only defined for powers of integers.
6.4 Exponents as power function:
The Rules of exponentiation are:
Domain: all numbers.
Range: all positive numbers.
If a (the base > 1, exponent is an increasing function, it is less than 1 for all negative x, and above 1 for all positive x. If a < 1, it is a decreasing function, greater than one for x < 0, and less than 1 for x greater than or equal to 0.
The equation involving a power function is an equation of the form
kxp = b.
The solution of such an equation, if a solution exists, is given by
x = (b/k)(1/p).
and it can be easily checked using the rules for exponentiation.
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