GMATWhizTeam
Bunuel
The circle defined by \(x^2 + y^2 - 6x - 10y + k = 0\) has no common point with any of the axis. If the point (1, 4) does not lie outside the circle, which of the following gives the range of all possible values of k ?
A. \(0 < k < 5\)
B. \(5 \leq k < 10\)
C. \(10 \leq k < 15\)
D. \(15 \leq k \leq 25\)
E. \(25 < k \leq 29\)
Are You Up For the Challenge: 700 Level QuestionsSolution
• Given circle’s equation \(= x^2 + y^2 – 6x – 10y + k = 0\)
o Or, \(x^2 – 2*3*x + 9 + y^2 – 2*5*y + 25 -9-25 + k = 0\)
o Or, \((x – 3)^2 + (y – 5)^2 = (\sqrt{34-k})^2\)
• Thus, the given circle has centre at (3, 5) and has a radius of \(\sqrt{34-k}\)
• Since, the circle doesn’t touch any of the axis, so radius of the circle must be smaller than the distance between its centre and the nearest axis.
o This means, \(\sqrt{34-k} < 3\)
Or, \(34 – k < 9\)
Or, \(k > 25 ………….(i)\)
• Also, it is given that the Point (1, 4) doesn’t lie outside the circle. So it must lie either inside the circle or on the circle.
• So, distance between centre of the circles and point (1, 4) must be less than or equal to the radius.
o i.e. \((1-3)^2 +(4-5)^2 ≤ 34 – k\)
o Or, \(4 + 1 ≤ 34 – k\)
o Or, \(k ≤ 29 ……………(ii)\)
• Combining inequalities (i) and (ii), we get ,
Thus, the correct answer is
Option D.hi,
GMATWhizTeamI really liked your explanation.
But I think there is a typo in your answer option.
Shouldn't it be E ?