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First of all, let's see what is the formula for area of triangle ADC. It is 1/2 * (base * height).
Base for triangle ADC is AC and height is BE. So Area of triangle ADC = 1/2 * (base * height) = 1/2 * (AC * BE). Well, the value for (AC * BE) is given which is 24. You got the area by simply putting the values of AC * BE in the formula. Area of triangle ADC is 12.
(Two ways to determine the height: either you draw a perpendicular line let's say DF, where F is the point on line l2 and now we got a square EBDF since lines l1 and l2 are parallel OR just take BE as the base...again DF and BE are same as we see that these two are the sides of the square EBDF, hence DF = BE)
I wish I could explain it by using some diagram or figures, then it would have been much easier for you to understand.
By the way, please write if you were able to understand it or not.
Last edited by Summer3 on 21 Feb 2007, 17:50, edited 3 times in total.