We define {{MATH_223}} to be the positive square-root, so that we can be sure that {{MATH_224}} is a function. Thinking of the square-root as the inverse of the squaring function, we can see the issue a little more clearly. There are two {{MATH_225}}-values that square to {{MATH_226}}. {{MATH_227}} Since we require that {{CMD_228}}square-root is a function{{CMD_229}}, we must have only one output value when we plug in {{MATH_230}}. We choose the positive square-root, meaning that {{MATH_231}} {{CMD_232}}