Let f (x) (x ∈ R) satisfy f (x1 * x2) = f (x1) + F (x2) for any real number x1, X2, and prove that f (x) is an even function
∵f(-1)=f(1*-1)=f(1)+f(-1)
∴f(1)=0
∵f(1)=f(-1*-1)=f(-1)+f(-1)
∴f(-1)=0
∴f(-x)=f(x*-1)=f(x)+f(-1)=f(x)
F (x) is an even function
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