Porposition5: @(因-(” ()(PTne)(图-(（么-）+(（：） proof: (1)trivial.(Hint:you can construct a bijection.) ·#料ak--(-）】 ·并ak-asog小-(（片)】 Combinating the two situations.then we prove(2). Porposition5: (1)  n k  =  n n − k  . (2) (Pascal Triangle)  n k  =  n − 1 k − 1  +  n − 1 k  . proof: (1) trivial.(Hint: you can construct a bijection.) (2) Separating  X k  into two parts. and find a fixed element t ∈ X. • #{all k−subsets containing t}=  n − 1 k − 1  . • #{all k−subsets avoiding t}=  n − 1 k  . Combinating the two situations.then we prove (2). 5