In the given figure, ABCD is a parallelogram and PD is parallel to QC.
S1 : △APD ≅ △BQC
S2 : area(△APD) + area(PBCD) = area(△BQC) + area(CDPB)
S1 and S2 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 AAS criteria
And area(△APD) = area(△BQC) (congruent triangles have equal areas)
Adding area(PBCD) on both sides of the equation
area(△APD) + area(PBCD) = area(△BQC) + area(PBCD)
Therefore, both S1 and S2 are true and S1 is the explanation of S2.