Let e1 and e2 be two distinct roots of the equation x2-ax+2=0. Let the sets \a R : e1 and e2 are the eccentricities of hyperbolas\=( , ), and \a R : e1 and e2 are the eccentricities of an ellipse and a hyperbola, respectively\=( , ). Then 2+ 2+ 2 is equal to :
- 18
- 22
- 26
- 34
Answer
(3) 26
Solution
Let h(x)=x2-ax+2; product of roots =2>0, sum =a
Ellipse : 0<e<1, hyperbola : e>1
Case 1 : both roots distinct and greater than 1
(i) D>0 a2-8>0 |a|>2 2
(ii) h(1)>0 3-a>0 a<3
(iii) vertex to the right of 1 : a2>1 a>2
a (2 2, 3 ) =2 2, =3
Case 2 : one root in (0, 1) and the other greater than 1
h(0)=2>0 and h(1)<0 3-a<0 a>3
( , )=(3, ) =3
2+ 2+ 2=8+9+9=26
Answer: (3)