In the given figure, ABCD is a parallelogram and PD is parallel to QC.
S1 : △APD≅△BQC
S2 : ar(△APD)+ ar(PBCD)= ar(△BQC)+ ar(CDPB)
S1 and S2 are are both true and S1 is the explanation for S2
In △APD and △BQC
∠PAD=∠QBC [corresponding angles]
AD=BC [opposite sides of parallelogram]
∠APD=∠BQC [corresponding angles]
△APD≅△BQC by ASA criteria
And ar(△APD)= ar(△BQC) [congruent triangles have equal areas]
Adding ar(PBCD) on both sides of the equation
ar(△APD)+ ar(PBCD)= ar(△BQC)+ ar(PBCD)
Therefore, both S1 and S2 are true and S1 is the explanation of S2.