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Easy

Question

If the roots of the quadratic equation x2 + 6x + b = 0 are real and distinct and they differ by atmost 4 then the least value of b is –

  1. 5
  2. 6
  3. 7
  4. 8

hintHint:

In the question, it is given that the roots of the quadratic equation x squared plus 6 x plus b space equals space 0 are real and distinct and they differ by atmost 4. We are required to find the least value of b.

The correct answer is: 5


    Step by step solution:
    Given equation: x squared plus 6 x plus b space equals space 0
    It is given that the roots of the equation are real and distinct.
    Therefore, B squared minus 4 A C space greater than space 0 space
    rightwards double arrow6 squared minus 4 left parenthesis 1 right parenthesis left parenthesis b right parenthesis space greater than space 0
    rightwards double arrow36 space minus space 4 b greater than 0
    rightwards double arrow 4 b less than 36
    rightwards double arrow b less than 9
    It is also given that the roots differ by atmost 4
    Therefore, open vertical bar alpha minus beta close vertical bar less or equal than 4
    rightwards double arrow open vertical bar alpha minus beta close vertical bar squared less or equal than 16
    rightwards double arrow open parentheses alpha plus beta close parentheses squared minus 4 alpha beta less or equal than 16
    rightwards double arrow open parentheses negative 6 close parentheses squared minus 4 left parenthesis b right parenthesis less or equal than 16      [ since sum of roots = -6 and product of roots = b]
    rightwards double arrow 36 minus 4 b less or equal than 16
    rightwards double arrow 4 b greater or equal than 20
    rightwards double arrow b greater or equal than 5
    Thus, least value of b is 5
    Hence, option(b) is the correct option.

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