504. 1) \(\left(a-\frac12\right)^2\); 2) \(\left(b+\frac13\right)^2\); 3) \(\left(x-\frac15\right)^2\).

1. \(\left(a-\frac12\right)^2=a^2-a+\frac14\);
2. \(\left(b+\frac13\right)^2=b^2+\frac23b+\frac19\);
3. \(\left(x-\frac15\right)^2=x^2-\frac25x+\frac1{25}\).

Ответ: \(a^2-a+\frac14;\ b^2+\frac23b+\frac19;\ x^2-\frac25x+\frac1{25}\).