Question

in a perfect square trinomial, the middle term must be twice the product of the of $a$ and $c$.

Explanation:

A perfect square trinomial has the form $a^2 \pm 2ab + b^2$, where the middle term is twice the product of the square roots of the first term ($a^2$) and the last term ($b^2$). Translating to the variables given ($a$ and $c$ for the first and last terms), the middle term equals $2\sqrt{a}\sqrt{c}$.

Answer:

square roots