SOLUTION: The equation {{{x^2-3x-2=0}}} has roots {{{alpha}}} and {{{beta}}}, and the equation {{{x^2-6x+p=0}}} has roots {{{k/alpha}}} and {{{k/beta}}}. find the value of {{{k}}} and of {{{

Algebra ->  Quadratic Equations and Parabolas -> SOLUTION: The equation {{{x^2-3x-2=0}}} has roots {{{alpha}}} and {{{beta}}}, and the equation {{{x^2-6x+p=0}}} has roots {{{k/alpha}}} and {{{k/beta}}}. find the value of {{{k}}} and of {{{      Log On


   



Question 173400: The equation x%5E2-3x-2=0 has roots alpha and beta, and the equation x%5E2-6x%2Bp=0 has roots k%2Falpha and k%2Fbeta. find the value of k and of p.
Answer by Edwin McCravy(20054) About Me  (Show Source):
You can put this solution on YOUR website!
The equation x%5E2-3x-2=0 has roots alpha and beta, and the equation x%5E2-6x%2Bp=0 has roots k%2Falpha and k%2Fbeta. find the value of k and of p.

We need to know two things about a quadratic equation of the
form x%5E2%2Bbx%2Bc=0

1.  matrix%281%2C2%2Cb%2C%22=%22%29 the sum of the two roots with the sign changed.

2.  matrix%281%2C2%2Cc%2C%22=%22%29 the product of the two roots.

We use 1 and 2 on the first equation:

Since x%5E2-3x-2=0 has roots alpha and beta,

Using 1, matrix%281%2C3%2C-3%2C%22=%22%2C-%28alpha%2Bbeta%29%29 
multiplying both sides by -1
matrix%281%2C3%2C3%2C%22=%22%2Calpha%2Bbeta%29

Using 2, matrix%281%2C3%2C-2%2C%22=%22%2Calpha%2Abeta%29 

Now we use 1 and 2 on the second equations:

Since x%5E2-6x%2Bp=0 has roots k%2Falpha and k%2Fbeta. 

Using 1, matrix%281%2C3%2C-6%2C%22=%22%2C-%28k%2Falpha%2Bk%2Fbeta%29%29 
multiplying both sides by -1
matrix%281%2C3%2C6%2C%22=%22%2Ck%2Falpha%2Bk%2Fbeta%29
getting the LCD of alpha%2Abeta



Combine numerators over the LCD:

matrix%281%2C3%2C6%2C%22=%22%2C%28k%2Abeta%2Bk%2Aalpha%29%2F%28alpha%2Abeta%29%29

Factor out k on top:

matrix%281%2C3%2C6%2C%22=%22%2C%28k%28beta%2Balpha%29%29%2F%28alpha%2Abeta%29%29

Now,

since we have above that matrix%281%2C3%2C3%2C%22=%22%2Calpha%2Bbeta%29,
and since %28beta%2Balpha%29=alpha%2Bbeta, we can replace
beta%2Balpha by 3,

and
since we have above that matrix%281%2C3%2C-2%2C%22=%22%2Calpha%2Abeta%29,
we can replace alpha%2Abeta by -2,

matrix%281%2C3%2C6%2C%22=%22%2C%28k%28beta%2Balpha%29%29%2F%28alpha%2Abeta%29%29
matrix%281%2C3%2C6%2C%22=%22%2C%28k%283%29%29%2F%28-2%29%29
matrix%281%2C3%2C6%2C%22=%22%2C-3k%2F2%29
Multiply both sides by 2
matrix%281%2C3%2C12%2C%22=%22%2C-3k%29
Divide both sides by -3
matrix%281%2C3%2C-4%2C%22=%22%2Ck%29

So the value of k is -4

-----------------

Using 2 on the second equation, 
matrix%281%2C3%2Cp%2C%22=%22%2C%28k%2Falpha%29%28k%2Fbeta%29%29%29 
But since k=-4,
matrix%281%2C3%2Cp%2C%22=%22%2C%28%28-4%29%2Falpha%29%28%28-4%29%2Fbeta%29%29%29
matrix%281%2C3%2Cp%2C%22=%22%2C16%2F%28alpha%2Abeta%29%29

Now since from above, we have matrix%281%2C3%2C-2%2C%22=%22%2Calpha%2Abeta%29,

matrix%281%2C3%2Cp%2C%22=%22%2C16%2F%28alpha%2Abeta%29%29 becomes

matrix%281%2C3%2Cp%2C%22=%22%2C16%2F%28-2%29%29

matrix%281%2C3%2Cp%2C%22=%22%2C-8%29

So the value of p is -8

Edwin