Nối DM ta có:

S(DMP)- S(DMN)= 3,5 (cm²) (1) (cùng cộng thêm S(DMO)Dễ thấy:

 S(DMN)= 1/6 S(ABCD)S(DAM)=1/6 S(ABCD)S(DPC)= 1/4 (S(ABCD)S(PMB)= (2/3 ×1/2 ×1/2) S(ABCD)= 1/6 S(ABCD)Vậy S(DMP)= 1- (1/6+1/4+1/6)=5/12 S(ABCD)Từ (1) suy ra:

5/12 S(ABCD)- 1/6 S(ABCD)

= 1/4 S(ABCD)= 3, 5 (cm²)Vây S(ABCD)= 14 cm².