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The number of ordered pairs (a,b) where ...

The number of ordered pairs `(a,b)` where a,b are integers satisfying the inequality min `(x ^(2) +(a-b) x + (1-a-b)) gtmax (-x ^(2) +(a+b)x-(1+a+b)AA x in R,` is :

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To solve the problem of finding the number of ordered pairs \((a, b)\) where \(a\) and \(b\) are integers satisfying the inequality \[ \min(x^2 + (a-b)x + (1-a-b)) > \max(-x^2 + (a+b)x - (1+a+b)) \] for all \(x \in \mathbb{R}\), we can follow these steps: ### Step 1: Identify the Quadratic Functions We have two quadratic functions: 1. \(f(x) = x^2 + (a-b)x + (1-a-b)\) 2. \(g(x) = -x^2 + (a+b)x - (1+a+b)\) ### Step 2: Determine the Nature of the Quadratics - The function \(f(x)\) is a **concave up** parabola (since the coefficient of \(x^2\) is positive). - The function \(g(x)\) is a **concave down** parabola (since the coefficient of \(x^2\) is negative). ### Step 3: Find the Minimum of \(f(x)\) and Maximum of \(g(x)\) - The minimum value of \(f(x)\) occurs at the vertex, given by: \[ x = -\frac{b}{2a} = -\frac{(a-b)}{2} \quad \text{(where \(a = 1\))} \] The minimum value is: \[ f\left(-\frac{(a-b)}{2}\right) = \left(-\frac{(a-b)}{2}\right)^2 + (a-b)\left(-\frac{(a-b)}{2}\right) + (1-a-b) \] Calculating this gives: \[ = \frac{(a-b)^2}{4} - \frac{(a-b)^2}{2} + (1-a-b) = \frac{(a-b)^2}{4} - \frac{2(a-b)^2}{4} + (1-a-b) = -\frac{(a-b)^2}{4} + (1-a-b) \] - The maximum value of \(g(x)\) also occurs at the vertex, given by: \[ x = -\frac{b}{2a} = \frac{(a+b)}{2} \quad \text{(where \(a = -1\))} \] The maximum value is: \[ g\left(\frac{(a+b)}{2}\right) = -\left(\frac{(a+b)}{2}\right)^2 + (a+b)\left(\frac{(a+b)}{2}\right) - (1+a+b) \] Calculating this gives: \[ = -\frac{(a+b)^2}{4} + \frac{(a+b)^2}{2} - (1+a+b) = \frac{(a+b)^2}{4} - (1+a+b) \] ### Step 4: Set Up the Inequality We need to satisfy the inequality: \[ -\frac{(a-b)^2}{4} + (1-a-b) > \frac{(a+b)^2}{4} - (1+a+b) \] ### Step 5: Simplify the Inequality Rearranging gives: \[ -\frac{(a-b)^2}{4} + 1 - a - b > \frac{(a+b)^2}{4} - 1 - a - b \] This simplifies to: \[ -\frac{(a-b)^2}{4} + 2 > \frac{(a+b)^2}{4} \] Multiplying through by 4 (to eliminate the fractions): \[ - (a-b)^2 + 8 > (a+b)^2 \] Rearranging gives: \[ 8 > (a+b)^2 + (a-b)^2 \] ### Step 6: Expand and Simplify Expanding the squares: \[ 8 > (a^2 + 2ab + b^2) + (a^2 - 2ab + b^2) = 2a^2 + 2b^2 \] This simplifies to: \[ 4 > a^2 + b^2 \] ### Step 7: Find Integer Solutions We need to find integer pairs \((a, b)\) such that: \[ a^2 + b^2 < 4 \] The possible integer pairs \((a, b)\) are: - \( (0, 0) \) - \( (0, 1) \) - \( (0, -1) \) - \( (1, 0) \) - \( (1, 1) \) - \( (1, -1) \) - \( (-1, 0) \) - \( (-1, 1) \) - \( (-1, -1) \) - \( (2, 0) \) - \( (0, 2) \) - \( (-2, 0) \) - \( (0, -2) \) However, since \(a^2 + b^2\) must be less than 4, the only valid pairs are: - \((0, 0)\) - \((0, 1)\) - \((0, -1)\) - \((1, 0)\) - \((1, 1)\) - \((1, -1)\) - \((-1, 0)\) - \((-1, 1)\) - \((-1, -1)\) ### Final Count Counting these valid pairs gives us a total of **9 ordered pairs**.
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VK JAISWAL ENGLISH-QUADRATIC EQUATIONS -EXERCISE (SUBJECTIVE TYPE PROBLEMS)
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