Finding the sides with equal angles

In the given figure BA is parallel to CD, . AB = 10cm, BC= 9cm, AC=15cm. What is the length of AD?

http://img200.imageshack.us/img200/328/trapezium.jpg

I tried solving with AB/BC = AD/AC. The answer turned out to be wrong because the solution in the book is given as,
Since ABC is similar to CAD, therefore, AB/BC = AC/AD.

I did not understand why they considered CAD instead DAC and AB/BC = AC/AD accordingly.

Quote:

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Originally Posted by saberteeth

I did not understand why they considered CAD instead DAC and AB/BC = AC/AD accordingly.

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In all cases involving similarly or congruence, the order in which we list the vertices in all important.
In this case it must be because .

Quote:

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Originally Posted by Plato

In all cases involving similarly or congruence, the order in which we list the vertices in all important.
In this case it must be because .

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Oh thanks, now i understand..