题目

There is one language that people use in every country in the world. People who use it are young and old, short and tall, thin and fat. It’s almost everybody’s second language. It’s easy to understand, but you can’t hear it. It’s sign language. Babies who can’t talk can point at things. They’re using sign language. Sign language can be useful in places that are very noisy and in places that are very quiet. You can talk to the people who are behind windows. And when you go swimming with your friends, you can have a“talk”under water! Many years ago, a Frenchman wanted to在计算“1×2+2×3+…+n（n+1）”时，某同学学到了如下一种方法：先改写第k项：由此得…相加，得类比上述方法，请你计算“1×2×3+2×3×4+…+n（n+1）（n+2）”，其结果为     ．