If a|b and a|c, then a|(b+c). If a2|b and b3|c, then a6|c. If a is an integer and a2|a, then a∈{−1,0,1}. If n∈N and n≥2, then...If a|b and a|c, then a|(b+c). If a2|b and b3|c, then a6|c. If a is an integer and a2|a, then a∈{−1,0,1}. If n∈N and n≥2, then the numbers n!+2,n!+3,n!+4,n!+5,⋯,n!+n are all composite. (Thus for any n≥2, one can find n−1 consecutive composite numbers. If a,b,c∈N and c≤b≤a, then (ab)(ac)=(ab−c)(a−b+cc).