In this problem, we will consider this system of simultaneous equations:
3x+5y−6z=2,(i)5xy−10yz−6xz=−41,(ii)xyz=6.(iii)
Let a=3x, b=5y, and c=-6z.
Given that (x,y,z) is a solution to the original system of equations, determine all distinct possible values of x+y.
I have forgotten how to do this.
I would like to be reminded of the method though. Any takers?
Thanks very much Alan,
I finally managed it with a different method.
3x+5y−6z=2,(i)5xy−10yz−6xz=−41,(ii)xyz=6.(iii)
Let
a=3x→x=a3b=5y→y=b5c=−6z→z=c−6
The above equations become
a+b+c=2ab+bc+ac=−123abc=−540
This gave me a hint:
If a,b and c are the roos of a monic cubic equations then we have.
Note: I used g because x has a different meaning in this question.
g3−(a+b+c)g2+(ab+bc+ac)g−(abcd)=0g3−(2)g2+(−123)g−(−540)=0g3−2g2−123g+540=0
I used the wolfram|alpha calculator to factoize this and got
(g−9)(g−5)(g+12)=0
so a,b,c = -12,5,9 (any order)
These can be arranged in 3! = 6 ways so there will be 6 answers.
To show all the possible answers I made up a Excel table:
Alan thinks that there are 9 solutions but i don't know what the others could be ....
Notice that my last solution is the one that Alan found.