13 Suppose that you want to choose a (2k -| 1)-element subset of the n- element set {1, 2, . You decide to do this by choosing rst the middle ele-...

Suppose that you want to choose a (2k + 1)-element subset of the n- element set {1, 2, . . . , n}. You decide by choosing first the middle element, then the k elements to its left, then the k elements to its right. Formulate the combinatorial identity you get from this.