you have to prove that ( a*b + 1 ) is not divisible by a or b, and the same for like (a*b*c*d*e*f*g + 1) is not divisible by a,b,c,d,e,f,g. (and of course they are not equal to one)
this is done really easily if you say that a*(...) ≡ 0 (mod a), then a*(...) + 1 ≡ 0+1 ≡ 1 (mod a). so if it is...