If the set of all a in mathbbR, for which the equation 2x^2 + (a - 5)x + 15 = 3a has no real root, is the interval (alpha, beta), and X = \x in mathbbZ : alpha < x < beta\, then sum_x in X x^2 is equal to

Solution & Explanation

### Related Formula For a quadratic equation Ax^2 + Bx + C = 0 to have no real roots, its discriminant must be strictly negative: D = B^2 - 4AC < 0 ### Core Logic Rearranging the given equation into standard quadratic form: 2x^2 + (a - 5)x + (15 - 3a) = 0 Here, A = 2, B = a - 5, and C = 15 - 3a. Setting the discriminant less than zero: (a - 5)^2 - 4(2)(15 - 3a) < 0 (a^2 - 10a + 25) - 8(15 - 3a) < 0 a^2 - 10a + 25 - 120 + 24a < 0 a^2 + 14a - 95 < 0 ### Step 1: Solve for the Interval Factorizing the quadratic inequality: (a + 19)(a - 5) < 0 Thus, a in (-19, 5). This gives alpha = -19 and \beta = 5. ### Step 2: Calculate the Sum of Squares The set X consists of integers strictly between -19 and 5: X = \-18, -17, dots, 0, 1, 2, 3, 4\ sum_x in X x^2 = (-18)^2 + (-17)^2 + dots + 4^2 = (1^2 + 2^2 + 3^2 + 4^2) + (1^2 + 2^2 + dots + 18^2) = frac4 times 5 times 96 + frac18 times 19 times 376 = 30 + 2109 = 2139 ### Pattern Recognition Recognize that the negative terms squared are identical to the positive terms squared. Splitting the summation avoids calculating large numbers manually or allows using standard formula templates like fracn(n+1)(2n+1)6 efficiently. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Mathematics: Quadratic Equations Class 11 Mathematics: Sequences and Series

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Q20 jee_main_2024_31_jan_morning Sign of Quadratic Expressions
Let S be the set of positive integral values of a for which fracax^2 + 2(a + 1)x + 9a + 4x^2 - 8x + 32 < 0, forall x in mathbbR. Then, the number of elements in S is:
  • A. 1
  • B. 0
  • C. infty
  • D. 3

Solution

### Core Logic For the denominator x^2 - 8x + 32, D = 64 - 128 < 0 and a = 1 > 0. Thus, x^2 - 8x + 32 > 0 forall x in mathbbR. ### Step 1: Constraint on Numerator Since the denominator is always positive, the numerator must be strictly negative for all x in mathbbR. ax^2 + 2(a + 1)x + 9a + 4 < 0 quad forall x in mathbbR This requires a < 0 and D < 0. ### Step 2: Conclusion Since a must be strictly less than 0, there are no *positive* integral values of a that satisfy the condition. Hence, S is an empty set. Number of elements is 0. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Maths: Quadratic Equations

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