In a rhombus ABCD, O is the meeting point of it's diagonals. If AB = 25 cm , OB = 7 cm  then find AO.

As, Diagonals of a Rhombus bisect each other at right angles.
Using Pythagoras theorem, 

In a right angled triangle AOB, the square on the hypotenuse is equal to the sum of the squares on the other two sides.

So, AO² + OB² = AB²

AO² + 7² = 25² 
625 - 49 = AO² 
576 = AO² 
AO = 24 cm