This topic should explore the implicit conditions of the topic.

We can only bring a non-negative number into the root sign, so we must judge the sign of a if we want to bring a into it. Because the square root of quadratic form must be non-negative, -A under the root sign of the original formula must be ≥0, so A≤0. Well, if you want to bring A in, you must leave a negative sign, so the original formula =-root sign (Party A *-A).