1. The first problem is to find the error and correct the expression $4 - 8$. 2. The expression $4 - 8$ means subtracting 8 from 4. 3. Calculate $4 - 8 = -4$. 4. There is no error in the subtraction itself; the result is $-4$. 5. The second problem asks why $6 \div \frac{1}{2}$ gives you a certain result. 6. Division by a fraction means multiplying by its reciprocal. 7. The reciprocal of $\frac{1}{2}$ is $2$. 8. So, $6 \div \frac{1}{2} = 6 \times 2 = 12$. 9. This is because dividing by a fraction less than 1 increases the value. 10. The "a" mentioned in the question seems incomplete or unclear, but mathematically, $6 \div \frac{1}{2} = 12$. 11. Regarding the diamond shape with no math labels, without more information or a graph, it cannot be analyzed mathematically here.