1.用數(shù)學歸納法證明"當n為正偶數(shù)為xn-yn能被x+y整除"第一步應驗證n=__________時,命題成立;第二步歸納假設(shè)成立應寫成_____________________.
2. 數(shù)學歸納法證明3能被14整除的過程中,當n=k+1時,3應變形為____________________.
3. 數(shù)學歸納法證明 1+3+9+…+3
4.求證 n能被9整除.