線代二版下冊-7-22範例7

Let v and w be two vectors in R^n. Suppose that ||v|| = 5 and ||w||=2. What are the smallest possible values of ||v-w|| and ||v+w||?

ans:
||v+(-w)|| >= | ||v|| - ||-w|| |
這是什麼定理呢??為什麼一定要符合這定理呢
麻煩各位了