Inactive Tutor answered 02/01/13
2x5/3 + 64 = 0
Subtract 64 from both sides of the equation:
2x5/3 = -64
Divide both sides of the equation by 2:
x5/3 = -32
Cube both sides of the equation:
(x5/3)3 = (-32)3
x5 = -32768
Take both sides of the equation to the power of 1/5:
(x5)1/5 = (-32768)1/5
x = -8
Inactive Tutor
How come? (-8)^(5/3) = -32
2(-32) + 64 = 0
I agree with Tamara.
There are no complex number solutions when there is an odd index (i.e. raising to the 1/5th is taking the 5th root) Taking an odd root of a negative number is just negative. Taking an even root of a negative number yields complex solutions.
03/30/13
Inactive Tutor
Greg, there still can be complex solutions when taking an odd root, there just happens to be a real root as well (the principal roots are complex): http://mathworld.wolfram.com/CubeRoot.html
eg. for x^5 = -32768 there are five roots: http://www.wolframalpha.com/input/?i=x%5E5+%3D+-32768
If you scroll down to the plot of the roots in the complex plane, you can see that there's one real root and four imaginary roots.
The number of roots will match the exponent - an exponent of n has n roots (whether n is even or odd, and whether the base is negative or positive), eg x^32 = 1 has 32 roots, two of them real (±1):
-8 is actually a solution to the equation, so I was wrong there.
03/31/13
Inactive Tutor
For x^32 = 1, you can see the plot of the roots: http://www.wolframalpha.com/input/?i=x%5E32+%3D+1+
03/31/13
Inactive Tutor
-8 is not a valid solution when you plug it back into the original equation
02/05/13