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Algebra Level 4

If $f(x)=\ln \left (\dfrac{1+x}{1-x} \right )$ then which of these answer choices is true?

f(x+2)-2f(x+1)+f(x)=0

f(x_1)\cdot f(x_2)=f(x_1+x_2)

f(x+1)+f(x)=f(x^2+x)

f(x_1)+f(x_2)=f \left (\frac{x_1+x_2}{1+x_1x_2} \right )

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