If the orthocenter of the triangle formed by the lines y = x + 1, y = 4x - 8 and y = mx + c is at (3, -1), then m - c is :
- 0
- 2
- -2
- 4
Answer
(1) 0
Solution
Vertex A (intersection of y = x + 1 and y = 4x - 8): x + 1 = 4x - 8 x = 3, y = 4
A(3, 4), orthocentre H(3, -1)
Altitude AH is the vertical line x = 3, so the opposite side y = mx + c is horizontal m = 0.
Third side: y = c. Vertex B on y = x + 1: B(c - 1, c)
Altitude from B is perpendicular to y = 4x - 8 slope of BH = - 14
c - (-1)(c - 1) - 3 = - 14 4c + 4 = -c + 4 c = 0
Check: C on y = 4x - 8 is (2, 0); slope of CH = 0 + 12 - 3 = -1, perpendicular to y = x + 1 ✓
m - c = 0 - 0 = 0
Answer: (1)