21 chia hết cho 2n+1
=> 2n+1 thuộc Ư(21) = {-21;-7; -3; -1;0; 1; 3; 7; 21}
2n+1 = -21 => n = -11
2n+1 = -7 => n = -4
2n+1 = -3 => n = -2
2n+1 = -1 => n = -1
2n+1 = 0 => n = -1/2
2n+1 = 1 => n = 0
2n+1 = 3 => n = 1
2n+1 = 7 => n = 3
2n+1 = 21 => n = 10
vậy n = {-11; -4; -2; -1; -1/2; 0; 1; 3; 10}