JEE quadratics 2

Algebra Level 4

If the maximum value of k k is A B \frac{-A}{B} for which k x 2 2 k x + 3 x 6 kx^{2}-2kx+3x-6 is positive for 2015 2015 integral values of x x .Find A + B A+B

This is a part of my set Some JEE problems .


The answer is 2021.

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2 solutions

Rushikesh Joshi
Apr 30, 2015

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

@Rushikesh Joshi How do you conclude that k k should be negative?

Ankit Kumar Jain - 4 years, 3 months ago
Aareyan Manzoor
Sep 28, 2015

z = k = a b z=-k=\frac{a}{b} note that what ever comes next is on the assumption that z > 0 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 \begin{aligned} -zx^2+2zx+3x-6>0\\ zx^2-2zx-3x+6<0\\ (zx-3)(x-2)<0\\ \end{aligned} so,from here we have 2 cases: { ( i ) 3 z < 2 3 z < x < 2 ( i i ) 3 z > 2 3 z > x > 2 \begin{cases} (i)\rightarrow \frac{3}{z}<2\rightarrow \frac{3}{z}<x<2\\ (ii)\rightarrow \frac{3}{z}>2\rightarrow \frac{3}{z}>x>2 \end{cases} note that the integral values for n: n 1 < n < n 2 n_1<n<n_2 ,is : n 2 n 1 1 n_2-n_1-1 .hence, { 3 z < x < 2 2 3 z 1 = 2015 z = 3 2014 ( r e j e c t e d ) 3 z > x > 2 3 z 2 1 = 2015 z = 3 2018 \begin{cases} \frac{3}{z}<x<2\rightarrow 2-\frac{3}{z}-1=2015\rightarrow z=-\frac{3}{2014}(rejected)\\ \frac{3}{z}>x>2\rightarrow \frac{3}{z}-2-1=2015\rightarrow z=\frac{3}{2018} \end{cases} this problem has nothing to do with maximum or minimum, a direct answer for z. 3 + 2018 = 2021 3+2018=\boxed{2021}

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