If the maximum value of k is B − A for which k x 2 − 2 k x + 3 x − 6 is positive for 2 0 1 5 integral values of x .Find A + B
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@Rushikesh Joshi How do you conclude that k should be negative?
z = − k = b a note that what ever comes next is on the assumption that z > 0 ,otherwise we will have to multiply by -1 which will ruin the inequity. − z x 2 + 2 z x + 3 x − 6 > 0 z x 2 − 2 z x − 3 x + 6 < 0 ( z x − 3 ) ( x − 2 ) < 0 so,from here we have 2 cases: { ( i ) → z 3 < 2 → z 3 < x < 2 ( i i ) → z 3 > 2 → z 3 > x > 2 note that the integral values for n: n 1 < n < n 2 ,is : n 2 − n 1 − 1 .hence, { z 3 < x < 2 → 2 − z 3 − 1 = 2 0 1 5 → z = − 2 0 1 4 3 ( r e j e c t e d ) z 3 > x > 2 → z 3 − 2 − 1 = 2 0 1 5 → z = 2 0 1 8 3 this problem has nothing to do with maximum or minimum, a direct answer for z. 3 + 2 0 1 8 = 2 0 2 1
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Observe one root is 2. Other root is -3/k obviously k should be negative .now,
the other root should lie between 2017 and 2018.we get k<-3/2018