Which of the following would be classified as a trinomial? 

 x2 + 2x 
 1 - x - x5 
 3xy 
 (xy)z 
 all of these 


A TERM, not counting its sign, is separated from 
other terms by either a plus or a minus sign.

If the sign before a term that separates the term from a term
before it is a plus sign, the term is positive.

If the sign before a term that separates the term from a term
before it is a minus sign, the term is negative.

If the term comes first in the expression and has no negative 
sign before it, it is positive.  If it comes first and has
a negative sign before it, then of course it is negative.

Let's go through the possible choices one by one:

x2 + 2x has TWO terms, x2 and 2x. 

1 - x - x5 has THREE terms, 1, -x, and -x5.

3xy has ONE term.

(xy)z has ONE term.

A TRIcycle has THREE wheels.  A TRInomial has THREE terms.

So the answer is the only one with THREE terms,

1 - x - x5

Edwin