Skip to main contentSkip to solution

The equation x3+(2r+1)x2+(4r−1)x+2=0x^3 + (2r + 1)x^2 + (4r - 1)x + 2 = 0 has −2- 2 as one of the roots. If the other two roots are real, then the minimum possible non-negative integer value of rr is

Entered answer:

Solution

✅ Correct Answer: 2

We need to find the value of rr when one root is −2-2 and the other two roots are real.

Let us work through this using Vieta's formulas - these connect the coefficients of a polynomial to sums and products of its roots.


For the cubic x3+(2r+1)x2+(4r−1)x+2=0x^3 + (2r + 1)x^2 + (4r - 1)x + 2 = 0, if the three roots are −2-2, α\alpha, and β\beta:

Product of all roots: (−2)⋅α⋅β=−21=−2(-2) \cdot \alpha \cdot \beta = -\frac{2}{1} = -2

This gives us: α⋅β=1\alpha \cdot \beta = 1

Sum of all roots: (−2)+α+β=−(2r+1)1=−(2r+1)(-2) + \alpha + \beta = -\frac{(2r + 1)}{1} = -(2r + 1)

This gives us: α+β=−2r−1+2=−2r+1\alpha + \beta = -2r - 1 + 2 = -2r + 1


Since α\alpha and β\beta are real numbers with α⋅β=1\alpha \cdot \beta = 1, we need to find when this is possible.

Key insight: When two real numbers have a positive product, they're either both positive or both negative.

If both are positive: Using AM-GM inequality, α+β≥2αβ=21=2\alpha + \beta \geq 2\sqrt{\alpha\beta} = 2\sqrt{1} = 2

If both are negative: Then α+β≤−2∣α∣∣β∣=−21=−2\alpha + \beta \leq -2\sqrt{|\alpha||\beta|} = -2\sqrt{1} = -2

Therefore: α+β≥2\alpha + \beta \geq 2 or α+β≤−2\alpha + \beta \leq -2


Substituting α+β=−2r+1\alpha + \beta = -2r + 1:

Case 1: −2r+1≥2-2r + 1 \geq 2

−2r≥1-2r \geq 1

r≤−12r \leq -\frac{1}{2}

Case 2: −2r+1≤−2-2r + 1 \leq -2

−2r≤−3-2r \leq -3

r≥32r \geq \frac{3}{2}

So: r≤−12r \leq -\frac{1}{2} or r≥32r \geq \frac{3}{2}


Since we need non-negative integer values, only r≥32r \geq \frac{3}{2} is relevant.

The minimum non-negative integer satisfying r≥32r \geq \frac{3}{2} is r=2r = 2.


Let us check r=2r = 2:

α+β=−2(2)+1=−3\alpha + \beta = -2(2) + 1 = -3

αβ=1\alpha \beta = 1

The quadratic with roots α,β\alpha, \beta is: t2−(−3)t+1=t2+3t+1=0t^2 - (-3)t + 1 = t^2 + 3t + 1 = 0

Discriminant =(−3)2−4(1)(1)=9−4=5>0= (-3)^2 - 4(1)(1) = 9 - 4 = 5 > 0

Since the discriminant is positive, both roots are real.

Therefore, the minimum possible non-negative integer value of rr is 2\boxed{2}.

Keyboard Shortcuts

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