A real-valued function f has the property that for all real numbers a and b,

f(a+b)+f(a-b)=2 f(a) f(b)

Which one of the following cannot be the value of f(1)?

**Answer Choices**

A. 0

B. 1

C. -1

D. 2

E. -2

A real-valued function f has the property that for all real numbers a and b,

f(a+b)+f(a-b)=2 f(a) f(b)

Which one of the following cannot be the value of f(1)?

**Answer Choices**

A. 0

B. 1

C. -1

D. 2

E. -2