F3Maths (triangles)

Let height of triangle from A to BD be h.
so area of triangle ABC = 2h/2
area of triangle ACD = 3h/2.
Now area of triangle ABC = 6, that is 2h/2 = 6, h = 6
so area of triangle ACD = 3h/2 = 9.
area ABD = area ABC + area ACD = 6 + 9 = 15 (D).




2009-08-15 18:14:57 補充：
Note that Area of triangle ABC : Area of triangle ACD = BC : CD. This is an important result.