Calculator can't do (-8)2/3
By Woon Khang Tang (P3742) on Wednesday, April 25, 2001 - 07:54 pm :

What is the answer for (-8) to the power of (2/3)?
The answer should be 4, but when I checked with my scientific calculator, the screen displayed "Ma error". Why? Is it that the calculator has errors or it's defined as unknown?

By Kerwin Hui (Kwkh2) on Wednesday, April 25, 2001 - 08:31 pm :
The answer is indeed 4. The problem here lies in the calculator's mechanism. For example, I can design a simple calculator so that, when you call the function xy, the following procedures are taken:
  1. Use the built-in function ln to find lnx
  2. Use another built-in function, the multiplication, to find ylnx.
  3. Now use another built-in function, ex to find eylnx, and display the answer.
 Using this calculator, you will experience problems when you do step 1, since the calculator does not know how to take the natural log of negative numbers, so an error message is displayed.

Here is another example: Try finding (-1)-3. I am not sure what model you are using so I can't predict what you will see. Here is a way to design a calculator that will not have error message in this calculation - build-in the formula (-1)x=cos(px) for integer x.

Kerwin

By Woon Khang Tang (P3742) on Wednesday, April 25, 2001 - 09:37 pm :

No problems for (-1)-3 with my calculator, the answer is -1. I'm using a Casio fx-570W scientific calculator. And in my pure maths text book, the curve of y = xn ×n is a ratio (p/q), the graph only exist if x is bigger or same as 0. Does it means that (-8)1/3 , (-8)2/3 is not computable? I'm sure that (-8)1/3 is -2 but I'm not sure of (-8)2/3 .

By Olof Sisask (P3033) on Thursday, April 26, 2001 - 05:32 pm :

Don't forget that (-8)2/3 = [(-8)1/3 ]2 , so if you're sure that (-8)1/3 = -2, then (-8)2/3 = (-2)2 = 4.

/Olof