18) 4a² + 1/4 b² + 2ab =

Rearrange the terms:

4a² + 2ab + 1/4 b²

Recognize the perfect square trinomial (a + b)² = a² + 2ab + b²:

(2a)² + 2(2a)(1/2 b) + (1/2 b)²

(2a + 1/2 b)²