Dumb question: What do brackets mean outside of matrices? Ex: [4+(9 - -x)]

and CR. math question, fill in the blanks with words to make a pun

the words:

addition, center, circumference, polyhedron, postulate, coincide, concentric, dimension, divide, rectangle, rhombus, secant, elliptical, factor, hypotenuse, subtract, multiply, plus, theorem.

heres the riddles:

A child's face will light up at _______ of candy

I bought my mother a book and ________ it by parcel post

A banana is sure to make an ______static

While giving his report, the nervous student became non__________ed

This _______ board consists of three layers

"Please, Rick, say yes. Won't you ___________?

thanks in advance