If –1 is a root of f(x), which of the following must be true?

a factor of f(x) is (x – 1).
a factor of f(x) is (x + 1).
both (x – 1) and (x + 1) are factors of f(x).
neither (x – 1) nor (x + 1) is a factor of f(x).

 For this case we have the following root:
 x=-1
 Imagine that the function f (x) is a polynomial of the form:
 
 To find the roots of the polynomial we equate to zero:
 
 You can factor the polynomial in the following way:
 
 The roots of the polynomial are:
 
 Therefore, in this case, if the root is x = -1 the following expression:
 x + 1
 It is a common factor in the function f (x).
 
 a factor of f (x) is (x + 1)