The focus of the parabola y2=4x+16 is the centre of the circle C of radius 5. If the values of , for which C passes through the point of intersection of the lines 3x-y=0 and x+ y=4, are 1 and 2, 1< 2, then 12 1+29 2 is equal to .
Answer
15
Solution
y2=4(x+4): vertex (-4,0), focus (-3,0). Circle C: (x+3)2+y2=25.
[figure]
Every line x+ y=4 passes through (4,0) and cuts y=3x; that point must lie on C: (x+3)2+9x2=25 5x2+3x-8=0 x=1, - 85
= 4-xy: points (1,3) =1; (- 85,- 245 ) =- 76
12 (- 76 )+29(1)=15
Answer: 15