cancel
Showing results for 
Search instead for 
Did you mean: 
agusb1
New member
2 1 0 0
Message 1 of 4
914
Flag Post

Solved!

Hp Prime displays incorrects values for function with cube root

HP Recommended
Hp Prime

When I see the table of values for this function 

agusb1_0-1582578268893.png

agusb1_1-1582578337450.png

It says that the function is not defined for values higher than 10. When I copy-paste and replace the X with 11, however, I do get a result.

agusb1_2-1582578421368.png

 

Also, trying different values that appeared defined in the table, gives different results:

agusb1_3-1582578517174.png

What is going? What is the real result?

 

 

Tags (1)
1 ACCEPTED SOLUTION

Accepted Solutions
cyrille
Level 6
Level 6
252 251 42 78
Message 4 of 4
Flag Post
HP Recommended

Hello,

 

Ok, got it...

Then, the answer si "simple"...

X^12 when X is 10 is 1e12... 1e12+4 is STILL 1e12 as the calculator only has 12 digit accuracy. take the cube root and get 1e4. X^4 with X=10 = 1e4, 1e4-1e4 = 0, you end up doing a divide by 0... which is undefined....

You are hitting the limit of what floating point arithmetic can do with 12 digit precision in decimal.

 

Now, let us move to the CAS side of the equation.

The CAS does NOT use decimal arithmetics, it uses binary arithmetics. It uses 44 bit of precision but only 11 bits of exponent.

This means that, to simplify, the CAS has a little bit more precision (14 digits from memory), but with a lesser range (the exponent is limited to + or - 308 vs the + or - 500 of the home/decimal system)...

As a result, this operation will still work in the CAS. In CAS, the problem will only apear a little bit later.

In cas, try 1e16+4 ENTER Ans-1e16 and you will get 0. Try 1e12+4 ENTER Ans-1e12 and you will get 4...

 

Cyrille

 

I am an HP Employee

View solution in original post

Tags (1)
Was this reply helpful? Yes No
3 REPLIES 3
cyrille
Level 6
Level 6
252 251 42 78
Message 2 of 4
Flag Post
HP Recommended

Hello,

 

This is definitely very strange. I get -0.5 here, which is the correct result.

cyrille_0-1582615161101.png

You can tell that it is correct as, when X gets big, the +4 in the X^12 becomes negligeable, so the X^12+4 is roughtly equal to X^12. The 3rd root of X^12 is X^4 (for X real). so you end up with the expression (at the bottom) X^4-3*X^4 = -2X^4.

You can simplify the top and bottom of your fraction and you get 1/-2 or 0.5 as the limit for your function when X gets big.

 

This is what I am getting on my prime.

 

What version of the FW are you using?

 

Cyrille

I am an HP Employee
Was this reply helpful? Yes No
agusb1
Author
New member
2 1 0 0
Message 3 of 4
Flag Post
HP Recommended

I think you misread the 3 from the cube root as a factor, the expression in the denominator is not x^4 - 3 * (x^4 +4) ^1/3 but just x^4 - (x^4 +4) ^1/3. The limit when x tends to infinity should be negative infinity. 

My software version is 2.1.14181 (2018 10 16)

Hardware version: D

 

Was this reply helpful? Yes No
cyrille
Level 6
Level 6
252 251 42 78
Message 4 of 4
Flag Post
HP Recommended

Hello,

 

Ok, got it...

Then, the answer si "simple"...

X^12 when X is 10 is 1e12... 1e12+4 is STILL 1e12 as the calculator only has 12 digit accuracy. take the cube root and get 1e4. X^4 with X=10 = 1e4, 1e4-1e4 = 0, you end up doing a divide by 0... which is undefined....

You are hitting the limit of what floating point arithmetic can do with 12 digit precision in decimal.

 

Now, let us move to the CAS side of the equation.

The CAS does NOT use decimal arithmetics, it uses binary arithmetics. It uses 44 bit of precision but only 11 bits of exponent.

This means that, to simplify, the CAS has a little bit more precision (14 digits from memory), but with a lesser range (the exponent is limited to + or - 308 vs the + or - 500 of the home/decimal system)...

As a result, this operation will still work in the CAS. In CAS, the problem will only apear a little bit later.

In cas, try 1e16+4 ENTER Ans-1e16 and you will get 0. Try 1e12+4 ENTER Ans-1e12 and you will get 4...

 

Cyrille

 

I am an HP Employee

View solution in original post

Tags (1)
Was this reply helpful? Yes No
† The opinions expressed above are the personal opinions of the authors, not of HP. By using this site, you accept the Terms of Use and Rules of Participation