ABCD is a trapezium with AB=CD.
Drop perpendiculars from B and C in the below diagram.
We have BE and CF perpendicular to AD.
Now in angle ABE and CFD
BE=CF (Altitude of trapezium)
AB=CD
[tex]\begin{gathered} \angle AEB=\angle DFC\text{ (right angle)} \\ From\text{ R.H.S congrugence} \\ \triangle ABE\cong\triangle CFD \\ \angle A=\angle D \\ Hence\text{ proved base angle of issoseles trapezium are equal.} \end{gathered}[/tex]
