A quadratic equation has only one distinct solution when it can be expressed as a perfect square trinomial, meaning it can be factored into a form like \((x - r)^2 = 0\).
Let's analyze each equation:
- \(x^2 + 5x + 4 = 0\): Factors to \((x + 4)(x + 1) = 0\) (two distinct solutions).
- \(x^2 - 4 = 0\): Factors to \((x - 2)(x + 2) = 0\) (two distinct solutions).
- \(x^2 + 4x + 4 = 0\): This factors to \((x + 2)^2 = 0\) (one distinct solution, \(x = -2\)).
- \(x^2 - 5x + 4 = 0\): Factors to \((x - 4)(x - 1) = 0\) (two distinct solutions).
The equation that has only one distinct solution is:
\(x^2 + 4x + 4 = 0\), which gives the solution \(x = -2\).