Question

How do you simplify `[cos(1/2)(phi)] x [cos(1/2)(phi)]`?

Answer 1

Perhaps
`color(white)("XXX")color(green)(cos(phi))`
(...but note the assumption that I made as to what was intended as the original expression)

Explanation:

If the expression was meant to be:
`color(white)("XXX")[cos^(1/2)(phi)]xx[cos^(1/2)(phi)]`
then since `k^a xx k^b= k^(a+b)`
we get the answer shown above.

...but maybe you meant
`color(white)("XXX")[cos(1/2) xx (phi)] xx [cos(1/2)xx(phi)]`
or something else... (???)