Multiple choice

If exactly one root of the quadratic equation ${ x }^{ 2 }-(k+\frac { 11 }{ 3 } )x-({ k }^{ 2 }+k+1)=0$ lies in (0, 3) then which one of the following relation is correct

  1. -3 < k < -1

  2. -8 < k < -4

  3. 1 < k < 4

  4. -7 < k < 5

Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

For exactly one root to lie in (0, 3), the product of the function values at the endpoints must be negative, f(0)*f(3) < 0. f(0) = -(k^2+k+1), f(3) = 9 - 3(k+11/3) - (k^2+k+1) = 9 - 3k - 11 - k^2 - k - 1 = -k^2 - 4k - 3. So (k^2+k+1)(k^2+4k+3) < 0. Since k^2+k+1 is always positive, we need k^2+4k+3 < 0, which is (k+1)(k+3) < 0, so -3 < k < -1.