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, so the given limit is 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)
x 2(x+p)=2+p=5 p=3
Equation: x2-3x+q=0; let h(x)=x2-3x+q. Both roots in (0, 2) requires:
(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) vertex x= 32 lies in (0, 2), true
q (2, 94 ] =2, = 94
4( + )=8+9=17
Answer: (3)