1. **Problem:** Calculate the value of $(-a)(+a)$. 2. **Formula:** Multiplying a negative and a positive number results in a negative number. Also, $(-a)(+a) = -a^2$. 3. **Work:** $$(-a)(+a) = -a^2$$ 4. **Explanation:** When you multiply $-a$ by $a$, you multiply the coefficients and add the exponents of $a$. Since $a$ is to the first power in both, the result is $a^2$, and the negative sign remains. **Final answer:** $-a^2$