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Find Parameter in Quadratic Equation Given Relations Between Roots

Let α,β\alpha, \beta be the roots of the equation x23x+r=0x^2 - 3x + r = 0, and α2,2β\frac{\alpha}{2}, 2\beta be the roots of the equation x2+3x+r=0x^2 + 3x + r = 0.

If the roots of the equation x2+6x=mx^2 + 6x = m are 2α+β+2r2\alpha + \beta + 2r and α2βr2\alpha - 2\beta - \frac{r}{2}, then mm is equal to :

Options

A

135-135

B

567-567

C

135135

D

567567

Correct

Step-by-Step Solution

Given the quadratic equation: x23x+r=0x^2 - 3x + r = 0

Since α\alpha and β\beta are the roots of this equation, by Vieta's formulas, we have: α+β=3— (1)\alpha + \beta = 3 \quad \text{--- (1)} αβ=r— (2)\alpha \beta = r \quad \text{--- (2)}

We are also given that α2\frac{\alpha}{2} and 2β2\beta are the roots of the equation: x2+3x+r=0x^2 + 3x + r = 0

Using Vieta's formulas for this equation: α2+2β=3    α+4β=6— (3)\frac{\alpha}{2} + 2\beta = -3 \implies \alpha + 4\beta = -6 \quad \text{--- (3)} (α2)(2β)=αβ=r(which is consistent with (2))\left(\frac{\alpha}{2}\right)(2\beta) = \alpha\beta = r \quad \text{(which is consistent with (2))}

Now, subtract equation (1) from equation (3): (α+4β)(α+β)=63(\alpha + 4\beta) - (\alpha + \beta) = -6 - 3 3β=9    β=33\beta = -9 \implies \beta = -3

Substituting β=3\beta = -3 into equation (1): α+(3)=3    α=6\alpha + (-3) = 3 \implies \alpha = 6

Using equation (2), we find rr: r=αβ=6×(3)=18r = \alpha \beta = 6 \times (-3) = -18

Now, let the roots of the equation x2+6xm=0x^2 + 6x - m = 0 be pp and qq, where: p=2α+β+2rp = 2\alpha + \beta + 2r q=α2βr2q = \alpha - 2\beta - \frac{r}{2}

Substituting the values of α\alpha, β\beta, and rr: p=2(6)+(3)+2(18)=12336=27p = 2(6) + (-3) + 2(-18) = 12 - 3 - 36 = -27 q=62(3)182=6+6+9=21q = 6 - 2(-3) - \frac{-18}{2} = 6 + 6 + 9 = 21

For the quadratic equation x2+6xm=0x^2 + 6x - m = 0, the product of the roots is given by m-m: pq=mp \cdot q = -m (27)×21=m(-27) \times 21 = -m 567=m    m=567-567 = -m \implies m = 567

Thus, the value of mm is 567567.

Find Parameter in Quadratic Equation Given Relations Between Roots | Mathematics PYQ Solution - JEE Challenger