при x = 0:
x * (9 − x) = 0 * (9 − 0) = 0;
21 − x = 21 − 0 = 21.
 
при x = 1:
x * (9 − x) = 1 * (9 − 1) = 1 * 8 = 8;
21 − x = 21 − 1 = 20.
 
при x = 2:
x * (9 − x) = 2 * (9 − 2) = 2 * 7 = 14;
21 − x = 21 − 2 = 19.
 
при x = 3:
x * (9 − x) = 3 * (9 − 3) = 3 * 6 = 18;
21 − x = 21 − 3 = 18.
 
при x = 4:
x * (9 − x) = 4 * (9 − 4) = 4 * 5 = 20;
21 − x = 21 − 4 = 17.
 
при x = 5:
x * (9 − x) = 5 * (9 − 5) = 5 * 4 = 20;
21 − x = 21 − 5 = 16.
 
при x = 6:
x * (9 − x) = 6 * (9 − 6) = 6 * 3 = 18;
21 − x = 21 − 6 = 15.
 
при x = 7:
x * (9 − x) = 7 * (9 − 7) = 7 * 2 = 14;
21 − x = 21 − 7 = 14.
 
при x = 8:
x * (9 − x) = 8 * (9 − 8) = 8 * 1 = 8;
21 − x = 21 − 8 = 13.
 
при x = 9:
x * (9 − x) = 9 * (9 − 9) = 9 * 0 = 0;
21 − x = 21 − 9 = 12.
 

Равенство верно при x = {3, 7}.