504. 1) \(\left(a-\frac12\right)^2\); 2) \(\left(b+\frac18\right)^2\); 3) \(\left(x-\frac15\right)^2\).

1. \(\left(a-\frac12\right)^2=a^2-a+\frac14\).
2. \(\left(b+\frac18\right)^2=b^2+\frac14b+\frac1{64}\).
3. \(\left(x-\frac15\right)^2=x^2-\frac25x+\frac1{25}\).

Ответ: \(a^2-a+\frac14;\ b^2+\frac14b+\frac1{64};\ x^2-\frac25x+\frac1{25}\).