Fill in the Blank with One of the Words or Phrases

Multiple Choice

Fill in the blank with one of the words or phrases listed below. $$\begin{array} { l l l } \text { index rationalizing } & \text { conjugate } & \text { principal square rootcube root midpoint } \\\text { complex numberlike radicals } & \text { radicand } & \text { imaginary unit } \quad \text { distance }\end{array}$$
-The_____ formula is $\left( \frac { x _ { 1 } + x _ { 2 } } { 2 } , \frac { y _ { 1 } + y _ { 2 } } { 2 } \right)$ .

A) conjugate
B) midpoint
C) cube root
D) distance