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