第6題改天再公佈答案...
7.The square PQRS is placed inside the square ABCD in such a way that the segments AP, BQ, CR and DS intersect neither each other nor the square PQRS. Prove that the sum of areas of quadrilaterals ABQP and CDSR is equal to the sum of the areas of quadrilaterals BCRQ and DAPS.