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So, sorry guys for reposting a question. But, it got deleted so I can't see it.

 

 

In this multi-part 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.  

 

(a) Determine the monic cubic polynomial in terms of a variable t whose roots are t=a, t=b, and t=c. Make sure your answer in terms of t and only t, in expanded form.

 

(b) Given that (x,y,z) is a solution to the original system of equations, determine all distinct possible values of x+y.

 Aug 3, 2016

Best Answer 

 #2
avatar+32633 
+16

Replace x, y and z in the equations by a, b and c:

 

a + b + c = 2

 

a*b/3 + b*c/3 + a*c/3 = -41.  So.  a*b + b*c + a*c = -123

 

-a*b*c/(3*5*6) = 6. So. a*b*c = -540

 

(t - a)(t - b)(t - c) = 0. ⇒ t^3 - (a + b + c)t^2 + (a*b + b*c + a*c)t - a*b*c = 0

 

Hence:  t^3 - 2t^2 - 123t + 540 = 0

 

Solving this gives: a = -12, b = 5, c = 9 or variants of these.

 

So:  x = a/3 → -4,   y = b/5 → 1,  z = -c/6 → -3/2

 

x + y = -3 This is just one possibility.  The others can be found by interchanging the values allocated to a, b and c.

 Aug 3, 2016
 #1
avatar+12384 
+4

Hello dabae!

 

3x+5y-6z=2 

5xy-10yz-6xz=-41  

xyz=6

 

x = 1,666666666667
y = 1,8
z = 2

 

Solved with computers for nonlinear equations.

Probably not the expected. Sorry!

 

Greeting asinus :- ) laugh !

 Aug 3, 2016
 #2
avatar+32633 
+16
Best Answer

Replace x, y and z in the equations by a, b and c:

 

a + b + c = 2

 

a*b/3 + b*c/3 + a*c/3 = -41.  So.  a*b + b*c + a*c = -123

 

-a*b*c/(3*5*6) = 6. So. a*b*c = -540

 

(t - a)(t - b)(t - c) = 0. ⇒ t^3 - (a + b + c)t^2 + (a*b + b*c + a*c)t - a*b*c = 0

 

Hence:  t^3 - 2t^2 - 123t + 540 = 0

 

Solving this gives: a = -12, b = 5, c = 9 or variants of these.

 

So:  x = a/3 → -4,   y = b/5 → 1,  z = -c/6 → -3/2

 

x + y = -3 This is just one possibility.  The others can be found by interchanging the values allocated to a, b and c.

Alan Aug 3, 2016

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