4. Prove the following statements: (a) If an integer is not divisible by 2 , then it cannot be divisible by 4 . Generalize this statement to all integers and their multiples. (b) If n is even, then n2+2n is divisible by 8 . (c) If a+b+c>0,ab+bc+ca>0, and abc>0, then a,b, and c are all positive