2.2: Rational Exponents
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Using Rational Exponents
Radical expressions can also be written without using the radical symbol. We can use rational (fractional) exponents. The index must be a positive integer. If the index
is even, then a cannot be negative.
We can also have rational exponents with numerators other than
In these cases, the exponent must be a fraction in lowest terms. We raise the base to a power and take an nth root. The numerator tells us the power and the denominator tells us the root.All of the properties of exponents that we learned for integer exponents also hold for rational exponents.
Rational exponents are another way to express principal nth roots. The general form for converting between a radical expression with a radical symbol and one with a rational exponent is
amn=(n√a)m=n√am
- Determine the power by looking at the numerator of the exponent.
- Determine the root by looking at the denominator of the exponent.
- Using the base as the radicand, raise the radicand to the power and use the root as the index.
Write 34323 as a radical. Simplify.
Solution
The 2 tells us the power and the 3 tells us the root.
34323=(3√343)2=3√3432
We know that 3√343=7 because 73=343. Because the cube root is easy to find, it is easiest to find the cube root before squaring for this problem. In general, it is easier to find the root first and then raise it to a power.
34323=(3√343)2=72=49
Write 47√a2 using a rational exponent.
Solution
The power is 2 and the root is 7, so the rational exponent will be 27. We get 4a27. Using properties of exponents, we get 47√2=4a−27
Simplify:
a. 5(2x34)(3x15)
b. (169)−12
Solution
a.
30x34x15 Multiply the coefficients
30x34+15 Use properties of exponents
30x1920 Simplify
(916)12 Use definition of negative exponents
√916 Rewrite as a radical
√9√16 Use the quotient rule
34 Simplify