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If a, b in R and ax^2 + bx +6 = 0,a!= 0 ...

If `a, b in R` and `ax^2 + bx +6 = 0,a!= 0` does not have two distinct real roots, then :

A

(a) minimum possible value of `3a+b` is `-2`

B

(b) minimum possible value of `3a+b` is 2

C

(c) minimum possible value of `6a+b` is `-1`

D

(d) minimum possible value of `6a+b` is 1

Text Solution

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The correct Answer is:
To solve the problem, we need to analyze the quadratic equation given by \( ax^2 + bx + 6 = 0 \) under the condition that it does not have two distinct real roots. ### Step-by-Step Solution: 1. **Understanding the Condition for Roots**: A quadratic equation \( ax^2 + bx + c = 0 \) has two distinct real roots if its discriminant \( D \) is greater than zero. The discriminant is given by: \[ D = b^2 - 4ac \] For our equation, \( c = 6 \), so we have: \[ D = b^2 - 4a \cdot 6 = b^2 - 24a \] 2. **Setting Up the Condition**: Since the equation does not have two distinct real roots, the discriminant must be less than or equal to zero: \[ b^2 - 24a \leq 0 \] This implies: \[ b^2 \leq 24a \] 3. **Rearranging the Inequality**: We can rearrange this inequality to express \( a \) in terms of \( b \): \[ a \geq \frac{b^2}{24} \] 4. **Finding the Minimum Value of \( 3a + b \)**: We want to find the minimum value of \( 3a + b \). Substituting \( a \) from the inequality into the expression: \[ 3a + b \geq 3\left(\frac{b^2}{24}\right) + b \] Simplifying this gives: \[ 3a + b \geq \frac{b^2}{8} + b \] 5. **Combining Terms**: To combine the terms, we can express \( b \) as \( \frac{8b}{8} \): \[ 3a + b \geq \frac{b^2 + 8b}{8} \] 6. **Completing the Square**: Now we complete the square for the expression \( b^2 + 8b \): \[ b^2 + 8b = (b + 4)^2 - 16 \] Therefore: \[ 3a + b \geq \frac{(b + 4)^2 - 16}{8} \] 7. **Finding the Minimum Value**: The minimum value of \( (b + 4)^2 \) is \( 0 \) (which occurs when \( b = -4 \)). Thus: \[ 3a + b \geq \frac{0 - 16}{8} = -2 \] 8. **Conclusion**: The minimum possible value of \( 3a + b \) is \( -2 \). ### Final Answer: The minimum possible value of \( 3a + b \) is \( -2 \).

To solve the problem, we need to analyze the quadratic equation given by \( ax^2 + bx + 6 = 0 \) under the condition that it does not have two distinct real roots. ### Step-by-Step Solution: 1. **Understanding the Condition for Roots**: A quadratic equation \( ax^2 + bx + c = 0 \) has two distinct real roots if its discriminant \( D \) is greater than zero. The discriminant is given by: \[ D = b^2 - 4ac ...
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