Solving function analytic expression by function equation method f(x)+f(3x)=3x What is the basis of F (3x) + F (x) = x

Solving function analytic expression by function equation method f(x)+f(3x)=3x What is the basis of F (3x) + F (x) = x

It's obviously wrong for you to convert in this way. That means 3x = x, then x = 0
It should be converted to f (x / 3) + F (x) = x, that is, if 3x is regarded as a whole, then x = 1 / 3 * 3x
If f (x) is a function of one variable, Let f (x) = KX + B, then there will be
KX+b+3kx+b=3x
KX + B + 1 / 3 * KX + B = x, so k = 3 / 4, B = 0, so f (x) = 3 / 4 * x