Common Root of Quadratic Equations

CAT 2025 Slot 2 · Quantitative Ability · Medium · Quadratic Equations

The equations 3x2−5x+p=03x^2 - 5x + p = 0 and 2x2−2x+q=02x^2 - 2x + q = 0 have one common root. The sum of the other roots of these two equations is

  1. A.

    \frac{8}{3} + p + \frac{q}{3}

  2. B.

    \frac{2}{3} - 2p + \frac{2q}{3}

  3. C.

    \frac{8}{3} - p + \frac{3q}{2}

  4. D.

    \frac{2}{3} - p + \frac{3q}{2}

Answer

C

Explanation

Let common root be α\alpha. For 3x2−5x+p=03x^2 - 5x + p = 0, sum of roots =α+β1=53  ⟹  β1=53−α= \alpha + \beta_1 = \frac{5}{3} \implies \beta_1 = \frac{5}{3} - \alpha. Product of roots =αβ1=p3  ⟹  α(53−α)=p3= \alpha \beta_1 = \frac{p}{3} \implies \alpha \left(\frac{5}{3} - \alpha\right) = \frac{p}{3}.

For 2x2−2x+q=02x^2 - 2x + q = 0, sum of roots =α+β2=1  ⟹  β2=1−α= \alpha + \beta_2 = 1 \implies \beta_2 = 1 - \alpha. Product of roots =αβ2=q2  ⟹  α(1−α)=q2= \alpha \beta_2 = \frac{q}{2} \implies \alpha(1 - \alpha) = \frac{q}{2}.

Subtracting products: α(53−α)−α(1−α)=p3−q2\alpha \left(\frac{5}{3} - \alpha\right) - \alpha(1 - \alpha) = \frac{p}{3} - \frac{q}{2} α(53−1)=23α=p3−q2  ⟹  α=p2−3q4\alpha \left(\frac{5}{3} - 1\right) = \frac{2}{3}\alpha = \frac{p}{3} - \frac{q}{2} \implies \alpha = \frac{p}{2} - \frac{3q}{4}

Sum of other roots β1+β2=(53−α)+(1−α)=83−2α\beta_1 + \beta_2 = \left(\frac{5}{3} - \alpha\right) + (1 - \alpha) = \frac{8}{3} - 2\alpha. Substituting α\alpha: β1+β2=83−2(p2−3q4)=83−p+3q2.\beta_1 + \beta_2 = \frac{8}{3} - 2\left(\frac{p}{2} - \frac{3q}{4}\right) = \frac{8}{3} - p + \frac{3q}{2}.

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