Let a variable line passing through the centre of the circle x2+y2-16x-4y=0, meet the positive co-ordinate axes at the points A and B. Then the minimum value of OA + OB, where O is the origin, is equal to
- 12
- 20
- 24
- 18
Answer
(4) 18
Solution
Centre (8, 2). Line through it: y - 2 = m(x - 8), with m < 0 so that it cuts both positive axes.
OA = 8 - 2m (x-intercept), OB = 2 - 8m (y-intercept)
S(m) = OA + OB = 10 - 8m - 2m
For the minimum, S'(m) = -8 + 2m2 = 0 m2 = 14
∵ m < 0, m = - 12
Since S''(m) = - 4m3 > 0 for m < 0, this is a minimum.
S = 10 + 4 + 4 = 18
Answer: (4)