Some of the steps in the derivation of the quadratic formula are shown. step 4: startfraction negative 4 a c + b squared over 4 a endfraction = a ( x + startfraction b over 2 a endfraction) squared step 5: (startfraction 1 over a endfraction) startfraction b squared minus 4 a c over 4 a endfraction = (startfraction 1 over a endfraction) a (x + startfraction b over 2 a endfraction) squared step 6: startfraction b squared minus 4 a c over 4 a squared endfraction = ( x + startfraction b over 2 a endfraction) squared step 7: startfraction plus or minus startroot b squared minus 4 a c endroot over 2 a endfraction = x + startfraction b over 2 a endfraction which best explains why the expression plus or minus startroot b squared minus 4 a c endroot cannot be rewritten as b plus or minus startroot negative 4 a c endroot during the next step? negative values, like −4ac, do not have a square root. the ± symbol prevents the square root from being evaluated. the square root of terms separated by addition and subtraction cannot be calculated individually. the entire term b2 − 4ac must be divided by 2a before its square root can be determined.