Prove Parallelogram Point P's Angles are Equal

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SUMMARY

The discussion focuses on proving that the angles $\angle PBA$ and $\angle PDA$ are equal in a parallelogram ABCD, given that point P is an interior point with $\angle PAB = \angle PCB$. By constructing lines P'C parallel to PB and P'P parallel to BC, two parallelograms, BCP'P and APP'D, are formed. The cyclic nature of points C, P'D, and P leads to the conclusion that $\angle DPP' = \angle DCP'$, which directly establishes the equality $\angle PBA = \angle PDA$.

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Albert1
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View attachment 972
Point P is an iner point of a parallelogram ABCD
given $\angle PAB=\angle PCB$
please prove :$\angle PBA=\angle PDA$
 

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View attachment 986
Construct P'C//PB , and P'P//BC ,connecting P'D
now
BCP'P and APP'D are two parallelograms
it is easy to see
$\angle P'DC=\angle PAB=\angle PCB=\angle P'PC$
four points C,P'D,P are cyclic
$\therefore \angle DPP'=\angle DCP'$
but $\angle PDA=\angle DPP' , and\,\, \angle DCP'=\angle PBA \therefore \angle PBA=\angle PDA$
 

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