Let A(1, 2) and C(-3, -6) be two diagonally opposite vertices of a rhombus, whose sides AD and BC are parallel to the line 7x - y = 14. If B( , ) and D( , ) are the other two vertices, then | + + + | is equal to :
- 3
- 6
- 1
- 9
Answer
(2) 6
Solution
Mid-point of AC: M(-1, -2); slope of AC = -6 - 2-3 - 1 = 2
Diagonals of a rhombus bisect each other at right angles BD has slope - 12 through M:
y + 2 = - 12(x + 1) ...(i)
AD through A with slope 7: y - 2 = 7(x - 1) y = 7x - 5 ...(ii)
Solving (i) and (ii): 7x - 3 = - x + 12
15x = 5 x = 13, y = - 83
D ( 13, - 83 )
B = 2M - D = (- 73, - 43 ) (check: slope BC = 7 ✓)
+ + + = - 73 - 43 + 13 - 83 = -6
| + + + | = 6
Answer: (2)