Skip to main contentSkip to solution

Let α\alpha and β\beta be the two distinct roots of the equation 2x2−6x+k=02x^2 - 6x + k = 0, such that (α+β)(\alpha + \beta) and αβ\alpha\beta are the distinct roots of the equation x2+px+p=0x^2 + px + p = 0. Then, the value of 8(k−p)8 (k - p) is

Entered answer:

Solution

✅ Correct Answer: 6

For any quadratic equation ax2+bx+c=0ax^2 + bx + c = 0, we have:

Sum of roots = −ba-\tfrac{b}{a}

Product of roots = ca\tfrac{c}{a}

If α\alpha and β\beta are roots, then (x−α)(x−β)=x2−(α+β)x+αβ=0(x - \alpha)(x - \beta) = x^2 - (\alpha + \beta)x + \alpha\beta = 0. Comparing with ax2+bx+c=0ax^2 + bx + c = 0, we get these relationships.


For our first equation 2x2−6x+k=02x^2 - 6x + k = 0:

a=2a = 2, b=−6b = -6, c=kc = k

Sum of roots: α+β=−(−6)2=62=3\alpha + \beta = -\frac{(-6)}{2} = \frac{6}{2} = 3

Product of roots: αβ=k2\alpha\beta = \frac{k}{2}


The second equation x2+px+p=0x^2 + px + p = 0 has roots (α+β)(\alpha + \beta) and αβ\alpha\beta.

We already know:

α+β=3\alpha + \beta = 3

αβ=k2\alpha\beta = \frac{k}{2}

So the roots of the second equation are 33 and k2\frac{k}{2}.


For the second equation x2+px+p=0x^2 + px + p = 0:

Sum of roots: 3+k2=−p3 + \frac{k}{2} = -p ... (equation 1)

Product of roots: 3×k2=p3 \times \frac{k}{2} = p ... (equation 2)


From equation 2: p=3k2p = \frac{3k}{2}

Substituting into equation 1:

3+k2=−3k23 + \frac{k}{2} = -\frac{3k}{2}


3=−3k2−k23 = -\frac{3k}{2} - \frac{k}{2}

3=−4k23 = -\frac{4k}{2}

3=−2k3 = -2k

k=−32k = -\frac{3}{2}


p=3k2=3(−32)2=−922=−94p = \frac{3k}{2} = \frac{3(-\frac{3}{2})}{2} = \frac{-\frac{9}{2}}{2} = -\frac{9}{4}


8(k−p)=8(−32−(−94))8(k - p) = 8\left(-\frac{3}{2} - \left(-\frac{9}{4}\right)\right)

=8(−32+94)= 8\left(-\frac{3}{2} + \frac{9}{4}\right)

Converting to common denominator: −32=−64-\frac{3}{2} = -\frac{6}{4}

=8(−64+94)= 8\left(-\frac{6}{4} + \frac{9}{4}\right)

=8(34)= 8\left(\frac{3}{4}\right)

=6= 6


When dealing with problems involving roots as coefficients of other equations, we always use the sum and product of roots formulas. This creates a system of equations that can be solved systematically.

Answer: 8(k−p)=68(k - p) = 6

Keyboard Shortcuts

  • Left arrow: Previous question
  • Right arrow: Next question
  • S key: Jump to solution
  • Q key: Jump to question