Let one root of the quadratic equation in x : (k2-15k+27 )x2+9(k-1)x+18=0 be twice the other. Then the length of the latus rectum of the parabola y2=6kx is equal to :
- 4
- 6
- 8
- 12
Answer
(4) 12
Solution
Let the roots be and 2 ; put p=k2-15k+27 (p 0)
Sum : 3 =- 9(k-1)p =- 3(k-1)p
Product : 2 2= 18p
2 9(k-1)2p2= 18p (k-1)2=p
k2-2k+1=k2-15k+27
13k=26 k=2
Check : p=4-30+27=1, equation x2+9x+18=0 has roots -3, -6 (one is twice the other)
Parabola : y2=12x=4(3)x a=3
Length of latus rectum =4a=12
Answer: (4)