Roots of Quadratic Equations

CAT 2023 Slot 1 · QA · Medium · Algebra

Let α\alpha and β\beta be the two distinct roots of the equation 2x26x+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(kp)8(k - p) is

Answer

6

Explanation

For 2x26x+k=02x^2 - 6x + k = 0, the sum and product of roots are: α+β=3,αβ=k2\alpha + \beta = 3, \quad \alpha\beta = \frac{k}{2}

The roots of x2+px+p=0x^2 + px + p = 0 are 33 and k2\frac{k}{2}. Sum of roots: 3+k2=p    k2+p=33 + \frac{k}{2} = -p \implies \frac{k}{2} + p = -3 Product of roots: 3k2=p    k2=p33 \cdot \frac{k}{2} = p \implies \frac{k}{2} = \frac{p}{3}

Substitute k2=p3\frac{k}{2} = \frac{p}{3} into the sum equation: p3+p=3    4p3=3    p=94\frac{p}{3} + p = -3 \implies \frac{4p}{3} = -3 \implies p = -\frac{9}{4} Then k=2p3=2(34)=32k = 2 \cdot \frac{p}{3} = 2 \cdot \left(-\frac{3}{4}\right) = -\frac{3}{2}.

Now evaluate 8(kp)8(k - p): kp=32(94)=34k - p = -\frac{3}{2} - \left(-\frac{9}{4}\right) = \frac{3}{4} 8(kp)=8×34=68(k - p) = 8 \times \frac{3}{4} = 6

Practise this under exam conditions

Sign in to solve it with a live timer, the on-screen CAT calculator, and streak and accuracy tracking across every question you attempt.

Solve in the workspace