math expression gives two diffrent answers (by hand and by mathomatic)
 

hello ...
im using mathomatic in my IT studies.

debian:/home/mha13# apt-cache policy mathomatic
mathomatic:
Installed: 12.5.16-1
Candidate: 12.5.16-1
Version table:
*** 12.5.16-1 0
500 http://mirror.hamakor.org.il testing/main Packages
500 http://mirror.hamakor.org.il unstable/main Packages
100 /var/lib/dpkg/status

i was checking my homework and i found something very odd :

in the equsion (from package mathomatic):

(a^2) | (18 + (2*(a^2))) | 3*a*((a^2) + a + 15)
#9: y = ------- | + | --------------------------- | -| --------------------
(a - 3) | ((a^2) + (3*a) + 9) | ((a^3) - 27)

if run the function simplify ill get y=2 + a

the issue is that if answer this by hand i get other result :



(a^2) (18 + (2*(a^2))) 3*a*((a^2) + a + 15)
#9: y = ------- + -------------------- - ------------------
(a - 3) ((a^2) + (3*a) + 9) ((a^3) - 27)


using the formula a^3 - b^3= (a-b)(a^2 + ab + b^2)


there for a^3 -27 = ((a^2) + (3*a) + 9) * (a - 3)



(a^2) *((a^2) + (3*a) + 9) + (18 + (2*(a^2)))*(a-3) - 3*a*((a^2) + a + 15)
= ----------------------------------------------------------------------
((a^3) - 27)



the strange issue is that upper expression should be
= a^4 - 3a^2 + 9a^2 + 18a + 2a^3 -54 -6a^2 -3a^3 - 3a^2 -45a
= a4 - a^3 +3a^2 - 27a - 54.

but when i put in mathomatic the expression :

(a^2) *(a^2 + 3*a + 9) + (18 + 2*a^2)*(a-3) - 3*a*a^2 + a + 15)

ill get :

#13: y = ((a^2)*((a^2) + (3*a) + 9)) + ((18 + (2*(a^2)))*(a - 3)) - (3*a*((a^2) + a + 15))

13-> simplify

#13: y = (a^4) + (2*(a^3)) - (27*a) - 54


where did the a^3 went away ?

p.s

i know that it is 6 grade issues but still i can't figure it out ..

Yours Truley

i saw that my msh could be messed up couse of font size so :

here is the orignal equsion.
http://img388.imageshack.us/img388/4562/1ps0.png



here is what mathomatic gave me for the upper equsion
http://img159.imageshack.us/img159/6228/2dc0.png

Well, my old work horse, the TI-89, tells me that

a^3 - b^3 = (a - b)(a^2 + ab + b^2)

Seems you've got a sign wrong :)

oops ..

thnx but it was a misstype (you can that i used the correct formule in the pictuers and in )
still it won't answer for me .

found the answer :
its ain't -3a^2 it is 3a^3 ..

god im bad in algebra ..

It's really easy to make these kind of mistakes, like sign errors or getting a power wrong, so don't worry about it. That's why one has to practice; math is a craft :)