SEAMO 2017 — Paper D — Lớp 7–8 Câu hỏi 8

Tính $\left(\frac23+\frac34+\cdots+\frac{49}{50}\right)\left(\frac12+\frac23+\cdots+\frac{48}{49}\right)-\left(\frac12+\frac23+\cdots+\frac{49}{50}\right)\left(\frac23+\frac34+\cdots+\frac{48}{49}\right)$.