A rod of length eight units moves such that its ends A and B always lie on the lines x-y+2=0 and y+2=0, respectively. If the locus of the point P, that divides the rod AB internally in the ratio 2:1 is 9 (x2+ y2+ xy+ x+28y )-76=0, then - - is equal to :
- 24
- 22
- 21
- 23
Answer
(4) 23
Solution
Let A(a,a+2), B(b,-2) and P(h,k) with AP:PB=2:1.
[figure]
h= a+2b3, k= a-23 a=3k+2, b= 3h-3k-22; put in AB2=(a-b)2+(a+4)2=64
9 [(3k-h+2)2+4(k+2)2 ]=256 9 (h2+13k2-6hk-4h+28k )-76=0
=13, =-6, =-4 - - =23
Answer: (4)