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 ( opp. 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)
⇒ar(△APD) + ar(PBCD) = ar(△BQC) + ar(CDPB)
Therefore, both S1 and S2 are true and S1 is the explanation of S2.