Bertushttp://www.allaboutcircuits.com/vol_3/chpt_1/7.html
T-section attenuator
ZIN = 25.97 + (35.14 ||(25.97 + 50)) & Numbers in figure are not same. Maybe same result of 50, but logically numbers in figure precede the numbers above. Number 25.97 cannot be deduced from 26.