find the formula for Un, the genereal term of (2/3) , (4/5) , (6/7) , (8/9)
Let me give you an idea, the denominator is always one greater than the numerator. (i.e: Numerator+1=Denominator) Therefore when you are writing the general term if the numerator is n then the denominator should be n+1. Now consider what the first term of the sequence is, that is when n=1 the numerator and denominator must be 2 and 3 respectively.
Hope you can continue.