1. **State the problem:** Calculate the sum of 1 and 2. 2. **Formula used:** Addition of two numbers is given by $a + b$. 3. **Apply the formula:** Here, $a = 1$ and $b = 2$, so we calculate $1 + 2$. 4. **Perform the addition:** $1 + 2 = 3$. 5. **Conclusion:** The sum of 1 and 2 is 3.