1) b) {1,2}; {1,3}; [1,4}; {2,3}; {2,4}; {3,4}
c) {1,2,3}; {1,3,4}; {2,3,4}
3) a) n!/(n-2)!=12 => [ (n-2)!(n-1)n ] /(n-2)!=12 => n(n-1)=12
=> n^2-n-12=0.....
b) => 22n!/n!=(n-3)!/(n-4)! => 22=n-3 ...
c) 6n!/n!= (n-3)!/(n-5)! => 6=(n-4)(n-3) => 6=n^2-7n+12 => n^2-7n+6=0....
4) a) => [(n-3)!(n-2)(n-1)] /(n-3)! <=20 => (n-2)(n-1)<=20 => n^2-3n-18<=0....
b)=> 16n!/5n! >(n-1)!/(n-2)! => 16/5>n-1....
c) (n-4)!/ [(n-4)!(n-3)(n-2)] >=1/20 => 1/[ (n-3)(n-2)]>=1/20 => (n-3)(n-2) >=20...