Common Root of Quadratic Equations

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

This is a medium Quantitative Ability question from the CAT 2025 Slot 2 paper. It tests Quadratic Equations. The full answer key and a step-by-step explanation are below — try it yourself first, then reveal the solution.

The equations 3x25x+p=03x^2 - 5x + p = 0 and 2x22x+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 3x25x+p=03x^2 - 5x + p = 0, sum of roots =α+β1=53    β1=53α= \alpha + \beta_1 = \frac{5}{3} \implies \beta_1 = \frac{5}{3} - \alpha. Product of roots =αβ1=p3    α(53α)=p3= \alpha \beta_1 = \frac{p}{3} \implies \alpha \left(\frac{5}{3} - \alpha\right) = \frac{p}{3}.

For 2x22x+q=02x^2 - 2x + q = 0, sum of roots =α+β2=1    β2=1α= \alpha + \beta_2 = 1 \implies \beta_2 = 1 - \alpha. Product of roots =αβ2=q2    α(1α)=q2= \alpha \beta_2 = \frac{q}{2} \implies \alpha(1 - \alpha) = \frac{q}{2}.

Subtracting products: α(53α)α(1α)=p3q2\alpha \left(\frac{5}{3} - \alpha\right) - \alpha(1 - \alpha) = \frac{p}{3} - \frac{q}{2} α(531)=23α=p3q2    α=p23q4\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=(53α)+(1α)=832α\beta_1 + \beta_2 = \left(\frac{5}{3} - \alpha\right) + (1 - \alpha) = \frac{8}{3} - 2\alpha. Substituting α\alpha: β1+β2=832(p23q4)=83p+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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