A variable line L passes through the point (3, 5) and intersects the positive coordinate axes at the points A and B. The minimum area of the triangle OAB, where O is the origin, is :
- 35
- 25
- 30
- 40
Answer
(3) 30
Solution
[figure]
L: xa + yb = 1 (a, b > 0) passes through P(3, 5), so 3a + 5b = 1
By A.M. G.M.: 1 = 3a + 5b 2 15ab ab 60
Area = 12ab 30, with equality when 3a = 5b = 12, i.e. a = 6, b = 10 (P is the mid-point of AB)
Minimum area = 30
Answer: (3)