Prove/Show that if ab and ba, where a and b are integers, then a=b or a=b. (20 points) Prove that if a,b,c, and d are integers, where a=0 and b=0, such that ac and bd, then abcd. (20 points) When x is an integer, prove that 1+x2(1+x)2 is true or disprove it by providing a counterexample. (20 points) Prove that if a is an integer that is not divisible by 3 , denoted as 3a, then (a+1)(a+2) is divisible by 3 , denoted as 3(a+1)(a+2). ( 24 points)