11 Line 8 states that \(\frac { a + b } { 2 } \geqslant \sqrt { a b }\) for \(a\), \(b \geqslant 0\). Explain why the result cannot be extended to apply in each of the following cases.
- One of the numbers \(a\) and \(b\) is positive and the other is negative.
- Both numbers \(a\) and \(b\) are negative.