# CAS question

Jozef Dobos shared this question 8 months ago

Hallo,

I do not understand why in CAS window

sqrt(x - 1) * sqrt(x - 2)

equals

-sqrt(x^2 - 3 * x + 2) ?

blondyn

1

Looks like a bug.I've looped your question to the Developers

1

not only in CAS . Try Simplify(sqrt(x - 1) sqrt(x - 2)) in algebra view

1

It's not as simple as it appears; -sqrt(x^2 - 3 * x + 2) is correct for some of the domain (for example x = 0)

We've made an improvement for the next release (v466) but there will still be "surprising" examples :)

If you need it just as a function then you can avoid the problems by using "Keep Input"

1

but the graph of f and simplify(f) has only one point in common

I thougth that GG suposse that the numbers are real in functions

1

`numbers are real in functions`
not in the CAS View

1

(and CAS commands eg Simplify)

1

and the Algebra View eg

`f(0 + 0ί)`

1

but the problem is in GG algebra view also

my graph is done only with algebra view

f(x)=sqrt(x-1) sqrt(x-2) (green graph)

simplify(f) (red graph)

1

Maple has not the problem.

1

then it seems to me that sqrt(x-1) sqrt(x-2)--->(sgn(x-1)+sgn(x-2))/2 sqrt(x^2-3x+2) because usually to simplify expands the domain

1

v466 online now

1

Dear Michael,

Thank you very much for the improvement. It is a bit better than before but

there are still a few issues to resolve. Please have a look at the

attachment I included previously: Maple_sqrt.pdf

I have tried also other CAS programs: Wolfram Alpha and MuPAD Light - they

all produce the same graphs as Maple.

I lecture about GeoGebra to high school teachers as part of a national

it, they even want to use it in their curriculum.

Unfortunately, we don't teach complex numbers at high schools here. Thus, we

have to find a suitable way how to use GeoGebra in teaching within these

limits.

I thank you once more for the great work you're doing for the community of

GeoGebra users.

Best,

Jozef

Files: v466.jpg
1

It's a similar problem to eg

`x/x`

You can use the Keep Input Tool to disable the automatic simplifying