Let x 2 ( (x-2) ) (rx2+(p-2)x-2p )(x-2)2=5 for some r, p R. If the set of all possible values of q, such that the roots of the equation rx2-px+q=0 lie in (0, 2), be the interval ( , ], then 4( + ) equals :
- 11
- 13
- 17
- 21
Answer
(3) 17
Solution
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x 2 (x - 2)x - 2 = 1, the given limit becomes x 2 rx2 + (p - 2)x - 2px - 2 = 5.
For a finite limit the numerator must vanish at x = 2:
4r + 2(p - 2) - 2p = 0 4r - 4 = 0 r = 1
Then x2 + (p - 2)x - 2p = (x - 2)(x + p), so
x 2(x + p) = 2 + p = 5 p = 3
The equation is x2 - 3x + q = 0; let h(x) = x2 - 3x + q. For both roots to lie in (0, 2) we need:
(i) D 0: 9 - 4q 0 q 94
(ii) h(0) > 0: q > 0
(iii) h(2) > 0: 4 - 6 + q > 0 q > 2
(iv) the vertex x = 32 lies in (0, 2), which is true.
Hence q (2, 94 ], so = 2 and = 94.
∴ 4( + ) = 8 + 9 = 17
Answer: (3)